diff --git a/lib/validations.js b/lib/validations.js
index 0b9388e7..bcf6a805 100644
--- a/lib/validations.js
+++ b/lib/validations.js
@@ -46,6 +46,20 @@ function Validatable() {
  */
 Validatable.validatesPresenceOf = getConfigurator('presence');
 
+/**
+ * Validate absence of one or more specified properties. 
+ * A model should not include a property to be considered valid; fails when validated field not blank.
+ *
+ * For example, validate absence of reserved
+ * ```
+ * Post.validatesAbsenceOf('reserved', { unless: 'special' });
+ *
+ * @param {String} propertyName  One or more property names.
+ * @options {Object} errMsg Optional custom error message.  Default is "can't be set"
+ * @property {String} message Error message to use instead of default.
+ */
+Validatable.validatesAbsenceOf = getConfigurator('absence');
+
 /**
  * Validate length. Require a property length to be within a specified range.
  * Three kinds of validations: min, max, is.
@@ -225,6 +239,15 @@ function validatePresence(attr, conf, err) {
   }
 }
 
+/*!
+ * Absence validator
+ */
+function validateAbsence(attr, conf, err) {
+  if (!blank(this[attr])) {
+    err();
+  }
+}
+
 /*!
  * Length validator
  */
@@ -332,6 +355,7 @@ function validateUniqueness(attr, conf, err, done) {
 
 var validators = {
   presence: validatePresence,
+  absence: validateAbsence,
   length: validateLength,
   numericality: validateNumericality,
   inclusion: validateInclusion,
@@ -470,8 +494,11 @@ function validationFailed(inst, v, cb) {
 
   // here we should check skip validation conditions (if, unless)
   // that can be specified in conf
-  if (skipValidation(inst, conf, 'if')) return false;
-  if (skipValidation(inst, conf, 'unless')) return false;
+  if (skipValidation(inst, conf, 'if') 
+    || skipValidation(inst, conf, 'unless')) {
+    if (cb) cb(true);
+    return false;
+  }
 
   var fail = false;
   var validator = validators[conf.validation];
@@ -500,7 +527,7 @@ function validationFailed(inst, v, cb) {
         message = 'is invalid';
       }
     }
-    inst.errors.add(attr, message, code);
+    if (kind !== false) inst.errors.add(attr, message, code);
     fail = true;
   });
   if (cb) {
@@ -533,6 +560,7 @@ function skipValidation(inst, conf, kind) {
 
 var defaultMessages = {
   presence: 'can\'t be blank',
+  absence: 'can\'t be set',
   length: {
     min: 'too short',
     max: 'too long',
diff --git a/test/validations.test.js b/test/validations.test.js
index a5d52ae6..1a3faf55 100644
--- a/test/validations.test.js
+++ b/test/validations.test.js
@@ -159,6 +159,20 @@ describe('validations', function () {
     });
 
   });
+  
+  describe('absence', function () {
+
+    it('should validate absence', function () {
+      User.validatesAbsenceOf('reserved', { if: 'locked' });
+      var u = new User({reserved: 'foo', locked: true});
+      u.isValid().should.not.be.true;
+      u.reserved = null;
+      u.isValid().should.be.true;
+      var u = new User({reserved: 'foo', locked: false});
+      u.isValid().should.be.true;
+    });
+
+  });
 
   describe('uniqueness', function () {
     it('should validate uniqueness', function (done) {
@@ -242,6 +256,18 @@ describe('validations', function () {
         });
       })).should.be.false;
     });
+    
+    it('should work with if/unless', function (done) {
+      User.validatesUniquenessOf('email', { 
+        if: function() { return true; },
+        unless: function() { return false; }
+      });
+      var u = new User({email: 'hello'});
+      Boolean(u.isValid(function (valid) {
+        valid.should.be.true;
+        done();
+      })).should.be.false;
+    });
   });
 
   describe('format', function () {
@@ -266,7 +292,38 @@ describe('validations', function () {
   });
 
   describe('custom', function () {
-    it('should validate using custom sync validation');
-    it('should validate using custom async validation');
+    it('should validate using custom sync validation', function() {
+      User.validate('email', function (err) {
+        if (this.email === 'hello') err();
+      });
+      var u = new User({email: 'hello'});
+      Boolean(u.isValid()).should.be.false;
+    });
+    
+    it('should validate and return detailed error messages', function() {
+      User.validate('global', function (err) {
+        if (this.email === 'hello' || this.email === 'hey') {
+          this.errors.add('hello', 'Cannot be `' + this.email + '`', 'invalid');
+          err(false); // false: prevent global error message
+        }
+      });
+      var u = new User({email: 'hello'});
+      Boolean(u.isValid()).should.be.false;
+      u.errors.should.eql({ hello: [ 'Cannot be `hello`' ] });
+    });
+    
+    it('should validate using custom async validation', function(done) {
+      User.validateAsync('email', function (err, next) {
+        process.nextTick(next);
+      }, { 
+        if: function() { return true; },
+        unless: function() { return false; }
+      });
+      var u = new User({email: 'hello'});
+      Boolean(u.isValid(function (valid) {
+        valid.should.be.true;
+        done();
+      })).should.be.false;
+    });
   });
 });