diff --git a/grails-app/controllers/uk/co/metadataconsulting/monitor/RecordCollectionController.groovy b/grails-app/controllers/uk/co/metadataconsulting/monitor/RecordCollectionController.groovy
index 5964800..a9ee420 100644
--- a/grails-app/controllers/uk/co/metadataconsulting/monitor/RecordCollectionController.groovy
+++ b/grails-app/controllers/uk/co/metadataconsulting/monitor/RecordCollectionController.groovy
@@ -185,7 +185,7 @@ class RecordCollectionController implements ValidateableErrorsMessage, GrailsCon
     }
 
     /**
-     * Validate a recordCollection with respect to its mapping which selects rules from the MDX.
+     * Validate a recordCollection with respect to its mapping, which is used to get validationRules from the MDX.
      * @param recordCollectionId
      */
     def validate(Long recordCollectionId) {
diff --git a/grails-app/controllers/uk/co/metadataconsulting/monitor/UrlMappings.groovy b/grails-app/controllers/uk/co/metadataconsulting/monitor/UrlMappings.groovy
index 4519552..33f6893 100644
--- a/grails-app/controllers/uk/co/metadataconsulting/monitor/UrlMappings.groovy
+++ b/grails-app/controllers/uk/co/metadataconsulting/monitor/UrlMappings.groovy
@@ -30,9 +30,9 @@ class UrlMappings {
 
         "/records/$recordCollectionId"(controller: 'record', action: 'index')
         "/record/index"(controller: 'record', action: 'index')
+        "/records/$recordCollectionId/$recordId"(controller: 'record', action: 'show')
         "/record/show"(controller: 'record', action: 'show')
         "/record/validate"(controller: 'record', action: 'validate')
-        "/records/$recordCollectionId/$recordId"(controller: 'record', action: 'show')
 
         "/login/$action?/$id?(.$format)?"(controller: 'login')
         "/login/$action?/$id?(.$format)?"(controller: 'logout')
diff --git a/grails-app/domain/uk/co/metadataconsulting/monitor/RecordPortionGormEntity.groovy b/grails-app/domain/uk/co/metadataconsulting/monitor/RecordPortionGormEntity.groovy
index d9e31c9..a7499ab 100644
--- a/grails-app/domain/uk/co/metadataconsulting/monitor/RecordPortionGormEntity.groovy
+++ b/grails-app/domain/uk/co/metadataconsulting/monitor/RecordPortionGormEntity.groovy
@@ -11,9 +11,12 @@ class RecordPortionGormEntity {
     String header
     String name
     String value
+
+    // The following fields (and the name field) get updated by a ValidationResult
     ValidationStatus status = ValidationStatus.NOT_VALIDATED
     String reason
     Integer numberOfRulesValidatedAgainst
+
     Date lastUpdated
 
     static belongsTo = [record: RecordGormEntity]
@@ -32,4 +35,15 @@ class RecordPortionGormEntity {
         reason type: 'text'
         sort 'header'
     }
+
+    @Override
+    String toString() {
+        """
+[header: ${header},
+name: ${name},
+value: ${value},
+status: ${status},
+reason: ${reason},
+numberOfRulesValidatedAgainst: ${numberOfRulesValidatedAgainst}]"""
+    }
 }
\ No newline at end of file
diff --git a/grails-app/services/uk/co/metadataconsulting/monitor/RecordCollectionService.groovy b/grails-app/services/uk/co/metadataconsulting/monitor/RecordCollectionService.groovy
index 025ace6..17add2d 100644
--- a/grails-app/services/uk/co/metadataconsulting/monitor/RecordCollectionService.groovy
+++ b/grails-app/services/uk/co/metadataconsulting/monitor/RecordCollectionService.groovy
@@ -28,6 +28,17 @@ class RecordCollectionService implements GrailsConfigurationAware {
 
     int pageSize
 
+    /**
+     * Validate a record collection with respect to:
+     * a headers map and
+     * a mapping from MDX CatalogueElements (identified by GORM URL) to validation rules.
+     *
+     * Done by validating each record individually/separately.
+     *
+     * @param recordCollectionId
+     * @param recordPortionMappingList
+     * @param validationRulesMap
+     */
     void validate(Long recordCollectionId, List<RecordPortionMapping> recordPortionMappingList, Map<String, ValidationRules> validationRulesMap) {
         int total = recordGormService.countByRecordCollectionId(recordCollectionId) as int
         for (int offset = 0; offset < total; offset = (offset + pageSize)) {
diff --git a/grails-app/services/uk/co/metadataconsulting/monitor/RecordService.groovy b/grails-app/services/uk/co/metadataconsulting/monitor/RecordService.groovy
index c34c650..1d0a4a8 100644
--- a/grails-app/services/uk/co/metadataconsulting/monitor/RecordService.groovy
+++ b/grails-app/services/uk/co/metadataconsulting/monitor/RecordService.groovy
@@ -43,11 +43,23 @@ class RecordService {
 
 
     @Transactional
+    /**
+     * Validate a record.
+     *
+     * Done by calling a method on each recordPortion which
+     * A. validates its value individually
+     * B. validates the whole record with validation rules relevant to that recordPortion's MDX CatalogueElement
+     *
+     * @param recordId the record being validated
+     *
+     */
     void validate(Long recordId, List<RecordPortionMapping> recordPortionMappingList, Map<String, ValidationRules> validationRulesMap) {
         DetachedCriteria<RecordGormEntity> query = recordGormService.queryById(recordId)
         query.join('portions')
         RecordGormEntity recordGormEntity = query.get()
+        log.info "ValidationRules Map: ${validationRulesMap.toString()}"
         for ( RecordPortionGormEntity recordPortionGormEntity : recordGormEntity.portions ) {
+            log.info "Validating record portion: ${recordPortionGormEntity.toString()}"
             ValidationResult validationResult = validateRecordPortionService.failureReason(recordPortionGormEntity,
                     recordPortionMappingList,
                     validationRulesMap)
diff --git a/grails-app/services/uk/co/metadataconsulting/monitor/ValidateRecordPortionService.groovy b/grails-app/services/uk/co/metadataconsulting/monitor/ValidateRecordPortionService.groovy
index 8735cc3..b7eac1f 100644
--- a/grails-app/services/uk/co/metadataconsulting/monitor/ValidateRecordPortionService.groovy
+++ b/grails-app/services/uk/co/metadataconsulting/monitor/ValidateRecordPortionService.groovy
@@ -10,17 +10,43 @@ class ValidateRecordPortionService {
     DlrValidatorService dlrValidatorService
     ValidatorService validatorService
 
+    /**
+     * Uses the GORM URL of a RecordPortion, found by the recordPortionMappingList, to find the relevant set of ValidationRules
+     * from the validationRulesMap.
+     * @param recordPortion
+     * @param recordPortionMappingList
+     * @param validationRulesMap
+     * @return
+     */
     ValidationRules validationRulesByRecordPortion(RecordPortionGormEntity recordPortion, List<RecordPortionMapping> recordPortionMappingList, Map<String, ValidationRules> validationRulesMap) {
         String recordPortionGormUrl = gormUrlByRecordPortionGormEntity(recordPortionMappingList, recordPortion)
         validationRulesMap.get(recordPortionGormUrl)
     }
 
+    /**
+     * Method that validates a recordPortion against
+     * A. associated MDX CatalogueElement (found by recordPortionMappingList)
+     * and B. ValidationRules associated with that MDX CatalogueElement (validationRulesMap)
+     *
+     * Actually validates in context of other related recordPortion-values-cells in the same record.
+     *
+     * @param recordPortion
+     * @param recordPortionMappingList
+     * @param validationRulesMap
+     * @return
+     */
     ValidationResult failureReason(RecordPortionGormEntity recordPortion, List<RecordPortionMapping> recordPortionMappingList, Map<String, ValidationRules> validationRulesMap) {
         ValidationRules validationRules = validationRulesByRecordPortion(recordPortion, recordPortionMappingList, validationRulesMap)
         RecordGormUrlsAndValues recordGormUrlsAndValues = recordGormUrlsAndValuesByRecordPortion(recordPortionMappingList, recordPortion)
         validatorService.validate(validationRules, recordPortion.value, recordGormUrlsAndValues)
     }
 
+    /**
+     * Essentially recovers the Record from the RecordPortion...
+     * @param recordPortionMappingList
+     * @param recordPortion
+     * @return
+     */
     RecordGormUrlsAndValues recordGormUrlsAndValuesByRecordPortion(List<RecordPortionMapping> recordPortionMappingList, RecordPortionGormEntity recordPortion) {
         List<String> gormUrls = []
         List<String> values = []
@@ -37,24 +63,36 @@ class ValidateRecordPortionService {
         }?.gormUrl
     }
 
-    String failureReason(ValidationRules validationRules, List<String> gormUrls, List<String> values) {
-        String reason
+    /**
+     * Method that executes the rules of "validationRules" in a Drools Rule Engine.
+     * The list of gormUrls and list of values should be of the same length;
+     * "values[i]" is the value in the cell in the column/header which is associated with the CatalogueElement identified by "gormUrls[i]".
+     * @param validationRules
+     * @param gormUrls
+     * @param values
+     * @return
+     */
+    String executeValidationRulesWithDrools(ValidationRules validationRules, List<String> gormUrls, List<String> values) {
+
         if ( !validationRules?.rules ) {
-            return reason
+            return ""
         }
+        List<String> droolsOutputs = []
         for ( ValidationRule validationRule : validationRules.rules ) {
 
+            // Create a mapping from identifiers (Drools global variables) to values to be validated.
+            // This makes up part of the "environment" against which the rule is executed
             Map m = [:]
             for ( String identifier : validationRule.identifiersToGormUrls.keySet() ) {
                 m[identifier] = valuesOfGormUrl(validationRule.identifiersToGormUrls[identifier], gormUrls, values)
             }
 
-            reason = dlrValidatorService.validate(validationRule.name, validationRule.rule, m)
-            if ( reason!=null ) {
-                break
-            }
+            droolsOutputs.add dlrValidatorService.validate(validationRule.name, validationRule.rule, m)
+
         }
-        reason
+
+        String concatenatedOutputs = droolsOutputs.join(',\n')
+        return "Validation Errors from Drools: [${concatenatedOutputs}]"
     }
 
     int indexOfGormUrl(List<String> gormUrls, String gormUrl) {
diff --git a/grails-app/services/uk/co/metadataconsulting/monitor/ValidatorService.groovy b/grails-app/services/uk/co/metadataconsulting/monitor/ValidatorService.groovy
index 0b9158e..ac270ab 100644
--- a/grails-app/services/uk/co/metadataconsulting/monitor/ValidatorService.groovy
+++ b/grails-app/services/uk/co/metadataconsulting/monitor/ValidatorService.groovy
@@ -8,19 +8,50 @@ import uk.co.metadataconsulting.monitor.modelcatalogue.ValidationRules
 class ValidatorService {
     ValidateRecordPortionService validateRecordPortionService
 
+    /**
+     * Validates a value against a validatingImpl
+     * and also validates the whole record that it comes from (represented by RecordGormUrlsAndValues),
+     * against a set of (Drools) validationRules.
+     *
+     * @param validationRules a set of validation rules, associated with the MDX Catalogue Element, associated with the value to be validated
+     * @param value value to be validated
+     * @param recordGormUrlsAndValues
+     * @return
+     */
     ValidationResult validate(ValidationRules validationRules, String value, RecordGormUrlsAndValues recordGormUrlsAndValues) {
+
         List<String> gormUrls = recordGormUrlsAndValues.gormUrls
         List<String> values = recordGormUrlsAndValues.values
-        String reason = validateRecordPortionService.failureReason(validationRules, gormUrls, values)
+
+        // Validate against Drools Validation Rules
+        String droolsOutput = validateRecordPortionService.executeValidationRulesWithDrools(validationRules, gormUrls, values)
+
         String name = validationRules?.name
         Integer numberOfRulesValidatedAgainst = validationRules?.rules?.size() ?: 0
 
+        String validatingImplOutput = ""
+        // Validate against the "ValidatingImpl"
         if ( validationRules?.validating ) {
             if ( !ValueValidator.validateRule(validationRules.validating, value) ) {
-                reason = reason ?: validationRules.validating.toString()
+                validatingImplOutput = validationRules.validating.toString()
             }
             numberOfRulesValidatedAgainst++
         }
+        String reason = ""
+        if (droolsOutput) {
+            if (validatingImplOutput) {
+                reason = "${droolsOutput}, Groovy Rule: [${validatingImplOutput}]"
+            }
+            else {
+                reason = droolsOutput
+            }
+
+        }
+        else if (validatingImplOutput) {
+            reason = validatingImplOutput
+        }
+
+
         ValidationStatus status = ValidationStatus.NOT_VALIDATED
         if ( numberOfRulesValidatedAgainst ) {
             status = reason ? ValidationStatus.INVALID : ValidationStatus.VALID
diff --git a/src/main/groovy/metadata/Validation.java b/src/main/groovy/metadata/Validation.java
index 72cd726..76b950e 100644
--- a/src/main/groovy/metadata/Validation.java
+++ b/src/main/groovy/metadata/Validation.java
@@ -13,15 +13,40 @@
 
 public class Validation {
 
-    public static int yearsBetween(Date last, Date first) {
-        Calendar a = getCalendar(first);
-        Calendar b = getCalendar(last);
-        int diff = b.get(YEAR) - a.get(YEAR);
-        if (a.get(MONTH) > b.get(MONTH) ||
-                (a.get(MONTH) == b.get(MONTH) && a.get(DATE) > b.get(DATE))) {
-            diff--;
+    /**
+     * Returns the difference in years between "last" and "first" (last-first),
+     * rounded DOWN (if negative, closer to 0) to the year.
+     * If the result is non-zero, then positive and negative are determined thus:
+     * If "last" is actually after "first", the result will be positive;
+     * otherwise negative.
+     *
+     * @param supposedlyLast
+     * @param supposedlyFirst
+     * @return
+     */
+    public static int yearsBetween(Date supposedlyLast, Date supposedlyFirst) {
+        int lastFirstComparison = supposedlyLast.compareTo(supposedlyFirst); // 1 if last > first, 0 if last == first, -1 if last < first
+        Calendar earlier = null;
+        Calendar later = null;
+
+        if (lastFirstComparison > 0) {
+            earlier = getCalendar(supposedlyFirst);
+            later = getCalendar(supposedlyLast);
+        }
+        else {
+            earlier = getCalendar(supposedlyLast);
+            later = getCalendar(supposedlyFirst);
         }
-        return diff;
+
+        int diff = later.get(YEAR) - earlier.get(YEAR);
+        if (diff != 0) {
+            if (earlier.get(MONTH) > later.get(MONTH) ||
+                    (earlier.get(MONTH) == later.get(MONTH) && earlier.get(DATE) > later.get(DATE))) {
+                diff--; // rounding down to the nearest year
+            }
+        }
+
+        return diff * lastFirstComparison; // multiply by comparison to get the right sign (plus or minus)
 
     }
 
diff --git a/src/main/groovy/uk/co/metadataconsulting/monitor/DlrValidator.java b/src/main/groovy/uk/co/metadataconsulting/monitor/DlrValidator.java
index ada1b11..fbb26f9 100644
--- a/src/main/groovy/uk/co/metadataconsulting/monitor/DlrValidator.java
+++ b/src/main/groovy/uk/co/metadataconsulting/monitor/DlrValidator.java
@@ -65,7 +65,7 @@ protected KieBase loadKnowledgeBaseFromString( KnowledgeBuilderConfiguration con
         }
 
         if (kbuilder.hasErrors()) {
-            //log.error(kbuilder.getErrors().toString());
+            log.error("Errors compiling rules. Rules: " + drlContentStrings.toString() + "\nErrors:" + kbuilder.getErrors().toString());
         }
         if (kBaseConfig == null) {
             kBaseConfig = KnowledgeBaseFactory.newKnowledgeBaseConfiguration();
diff --git a/src/main/groovy/uk/co/metadataconsulting/monitor/RecordGormUrlsAndValues.groovy b/src/main/groovy/uk/co/metadataconsulting/monitor/RecordGormUrlsAndValues.groovy
index 9d973a6..239e9c3 100644
--- a/src/main/groovy/uk/co/metadataconsulting/monitor/RecordGormUrlsAndValues.groovy
+++ b/src/main/groovy/uk/co/metadataconsulting/monitor/RecordGormUrlsAndValues.groovy
@@ -3,6 +3,12 @@ package uk.co.metadataconsulting.monitor
 import groovy.transform.CompileStatic
 
 @CompileStatic
+/**
+ * Partially represents a Record as a list of gormUrls and values.
+ * gormUrls[i] and values[i] should correspond to each other.
+ * gormUrls[i] is the GORM URL of the MDX Catalogue Element, associated with the column-header,
+ * under which is the cell, from which values[i] came.
+ */
 class RecordGormUrlsAndValues {
     List<String> gormUrls
     List<String> values
diff --git a/src/main/groovy/uk/co/metadataconsulting/monitor/RecordPortion.groovy b/src/main/groovy/uk/co/metadataconsulting/monitor/RecordPortion.groovy
index 2b14890..e17f940 100644
--- a/src/main/groovy/uk/co/metadataconsulting/monitor/RecordPortion.groovy
+++ b/src/main/groovy/uk/co/metadataconsulting/monitor/RecordPortion.groovy
@@ -5,6 +5,9 @@ import groovy.transform.CompileStatic
 
 @Canonical
 @CompileStatic
+/**
+ * non-GORM version of RecordPortionGormEntity (i.e. a "Cell" in a "Table")
+ */
 class RecordPortion {
     String header
     String name
diff --git a/src/main/groovy/uk/co/metadataconsulting/monitor/RecordPortionMapping.groovy b/src/main/groovy/uk/co/metadataconsulting/monitor/RecordPortionMapping.groovy
index 1009265..c931eeb 100644
--- a/src/main/groovy/uk/co/metadataconsulting/monitor/RecordPortionMapping.groovy
+++ b/src/main/groovy/uk/co/metadataconsulting/monitor/RecordPortionMapping.groovy
@@ -7,13 +7,27 @@ import uk.co.metadataconsulting.monitor.modelcatalogue.GormUrlName
 @ToString
 @CompileStatic
 /**
- * Non-GORM representation of RecordCollectionMappingEntryGormEntity.
+ * Non-GORM representation (i.e. class not in the Grails domain) of RecordCollectionMappingEntryGormEntity.
+ * Represents a mapping from a header to a MDX CatalogueElement (identified by gormUrl).
+ * Actually a bit poorly named since it's more to do with a "column (header)" than individual RecordPortions (cells).
  */
 class RecordPortionMapping {
     Long id
+    /**
+     * Header for the RecordCollection/DataSet
+     */
     String header
+    /**
+     * Identifies the associated MDX CatalogueElement
+     */
     String gormUrl
+    /**
+     * Name of the associated MDX CatalogueElement
+     */
     String name
+    /**
+     * Combines gormURL and name. This is sort of a hack for the front-end Javascript selection/search library to work properly
+     */
     String combinedGormUrlName
 
     static RecordPortionMapping of(RecordCollectionMappingEntryGormEntity gormEntity) {
diff --git a/src/main/groovy/uk/co/metadataconsulting/monitor/ValidationResult.groovy b/src/main/groovy/uk/co/metadataconsulting/monitor/ValidationResult.groovy
index 5b2d7e3..2c882f9 100644
--- a/src/main/groovy/uk/co/metadataconsulting/monitor/ValidationResult.groovy
+++ b/src/main/groovy/uk/co/metadataconsulting/monitor/ValidationResult.groovy
@@ -3,8 +3,17 @@ package uk.co.metadataconsulting.monitor
 import groovy.transform.CompileStatic
 
 @CompileStatic
+/**
+ * The result of a validation process
+ */
 class ValidationResult {
+    /**
+     * Represents the reason validation failed
+     */
     String reason
+    /**
+     * Name of the MDX CatalogueElement being validated
+     */
     String name
     int numberOfRulesValidatedAgainst = 0
     ValidationStatus status = ValidationStatus.NOT_VALIDATED
diff --git a/src/main/groovy/uk/co/metadataconsulting/monitor/modelcatalogue/ValidationRule.groovy b/src/main/groovy/uk/co/metadataconsulting/monitor/modelcatalogue/ValidationRule.groovy
index 1bbb7ad..ba803a3 100644
--- a/src/main/groovy/uk/co/metadataconsulting/monitor/modelcatalogue/ValidationRule.groovy
+++ b/src/main/groovy/uk/co/metadataconsulting/monitor/modelcatalogue/ValidationRule.groovy
@@ -5,8 +5,30 @@ import groovy.transform.CompileStatic
 
 @Canonical
 @CompileStatic
+/**
+ * Representation of a Drools Rule, which are Validation Rules/Business Rules in the MDX
+ */
 class ValidationRule {
+    /**
+     * Name of the ValidationRule
+     */
     String name
+    /**
+     * Mapping from the identifiers (global variables used in Drools Rules) to GORM URLs (which identify CatalogueElements in the MDX)
+     */
     Map<String, String> identifiersToGormUrls
+    /**
+     * Executable Drools Rule script
+     */
     String rule
+
+    @Override
+    public String toString() {
+        """
+ValidationRule[
+name: ${name},
+identifiersToGormUrls: ${identifiersToGormUrls.toString()},
+rule: ${rule}
+]"""
+    }
 }
diff --git a/src/main/groovy/uk/co/metadataconsulting/monitor/modelcatalogue/ValidationRules.groovy b/src/main/groovy/uk/co/metadataconsulting/monitor/modelcatalogue/ValidationRules.groovy
index e98e4cc..3be0168 100644
--- a/src/main/groovy/uk/co/metadataconsulting/monitor/modelcatalogue/ValidationRules.groovy
+++ b/src/main/groovy/uk/co/metadataconsulting/monitor/modelcatalogue/ValidationRules.groovy
@@ -4,11 +4,42 @@ import groovy.transform.CompileStatic
 import org.modelcatalogue.core.scripting.ValidatingImpl
 
 @CompileStatic
+/**
+ * An MDX CatalogueElement with A. an associated set of ValidationRules (used in Drools Engine),
+ * plus B. a "ValidatingImpl" (another kind of rule... Groovy script)
+ */
 class ValidationRules {
 
+    /**
+     * URL of MDX CatalogueElement
+     */
     String url
+    /**
+     * GORM URL of MDX CatalogueElement
+     */
     String gormUrl
+    /**
+     * Name of MDX CatalogueElement
+     */
     String name
+    /**
+     * Associated ValidationRules (Drools Rules)
+     */
     List<ValidationRule> rules = []
+    /**
+     * Another kind of validation rule. A Groovy Script.
+     */
     ValidatingImpl validating
+
+    @Override
+    String toString() {
+        String rulesString = rules.collect{it.toString()}.join(',\n')
+        """
+ValidationRules[url: ${url},
+gormUrl: ${gormUrl},
+name: ${name},
+rules: [${rulesString}],
+validating: ${validating.toString()}
+"""
+    }
 }