diff -u b/core/lib/Drupal/Core/Entity/EntityFormController.php b/core/lib/Drupal/Core/Entity/EntityFormController.php
--- b/core/lib/Drupal/Core/Entity/EntityFormController.php
+++ b/core/lib/Drupal/Core/Entity/EntityFormController.php
@@ -50,15 +50,20 @@
 
     // Retrieve the constraints as objects.
     $constraint_set = $entity->getConstraintsObjects();
-    // Loop all, if there's a matching form element, set #required.
+
+    // Loop all constraints.
     foreach ($constraint_set as $field_name => $constraints) {
+      // Handle field constraints.
       if (isset($form[$field_name]) && !empty($constraints['field'])) {
         foreach ($constraints['field'] as $constraint) {
+          // @todo: find a better way to handle this, or use constraints passed
+          // by the field / typedData
           if (!isset($form[$field_name][0])) {
             $form[$field_name] += $constraint->convertToFormAPI();
           }
         }
       }
+      // Handle typedData constraints.
       if (isset($form[$field_name]) && !empty($constraints[0])) {
         foreach ($constraints[0] as $constraint) {
           if (!isset($form[$field_name][0])) {
diff -u b/core/lib/Drupal/Core/Validation/Constraint/Constraint.php b/core/lib/Drupal/Core/Validation/Constraint/Constraint.php
--- b/core/lib/Drupal/Core/Validation/Constraint/Constraint.php
+++ b/core/lib/Drupal/Core/Validation/Constraint/Constraint.php
@@ -30,19 +30,42 @@
   protected $message_arguments = array();
 
   /**
-   * Settings of the constraint.
+   * Type of the constraint.
    *
    * @var string
+   *   Either '', 'entity', 'field' or 'typedData'
    */
-  protected $settings = array();
+  protected $type = '';
 
   /**
-   * Type of the constraint.
+   * The javascript rules.
    *
-   * @var string
-   *   Either '', 'entity', 'field' or 'typedData'
+   * @var array
+   *   Array containing all javascript rules to apply, keyed by ....
    */
-  protected $type = '';
+  protected $js_rules = array();
+
+  /**
+   * The (optional) custom javascript rule definitions.
+   *
+   * @var array
+   *   Array of rules keyed by short name, pointing to a js file for each rule.
+   */
+  protected $js_custom_rules = array();
+
+  /**
+   * The data definition.
+   *
+   * @var array
+   */
+  protected $definition = array();
+
+  /**
+   * The properties.
+   *
+   * @var array
+   */
+  protected $properties = array();
 
   /**
    * Constructs a new constraint.
@@ -50,21 +73,46 @@
    * @param array $definition
    *   Definition as specified in the plugin definition.
    *
-   * @param array $settings
-   *   Settings passed by the entity, field or typed data.
+   * @param array $properties
+   *   Properties passed by the entity, field or typed data.
    */
-  public function __construct(array $definition, array $settings) {
-    $this->settings = $settings;
+  public function __construct(array $definition, array $properties) {
+    $this->definition = $definition;
+
     if (isset($definition['message'])) {
       $this->message = $definition['message'];
     }
     if (isset($definition['type'])) {
-      $this->message = $definition['type'];
+      $this->type = $definition['type'];
+    }
+
+    // Get properties from plugin definition.
+    if (isset($definition['properties'])) {
+      $this->properties = $definition['properties'];
+    }
+
+    // Passed in properties override the definition properties.
+    if ($properties && is_array($properties)) {
+      $this->properties = $properties + $this->properties;
     }
   }
 
-  public function setSettings($settings) {
-    $this->settings = $settings;
+  /**
+   * Sets the properties to use.
+   *
+   * @param array $properties
+   */
+  public function setProperties($properties) {
+    $this->properties = $properties + $this->properties;
+  }
+
+  /**
+   * Gets the properties.
+   *
+   * @return array
+   */
+  public function getProperties() {
+    return $this->properties;
   }
 
   /**
@@ -112,2 +160,23 @@
 
+  /**
+   * Gets the javascript rules.
+   *
+   * @return array
+   *   Array containing all javascript rules to apply, keyed by ....
+   *   This data will get passed to clientside validation.
+   */
+  public function getJavascriptRules() {
+    return $this->$js_rules;
+  }
+
+  /**
+   * Gets the custom javascript rules.
+   *
+   * @return array
+   *   Array of rules keyed by short name, pointing to a js file for each rule.
+   *   This data will get attached to the form.
+   */
+  public function getJavascriptCustomRules() {
+    return $this->js_custom_rules;
+  }
 }
diff -u b/core/lib/Drupal/Core/Validation/Constraint/ConstraintFactory.php b/core/lib/Drupal/Core/Validation/Constraint/ConstraintFactory.php
--- b/core/lib/Drupal/Core/Validation/Constraint/ConstraintFactory.php
+++ b/core/lib/Drupal/Core/Validation/Constraint/ConstraintFactory.php
@@ -41,7 +41,7 @@
         $definition = $this->discovery->getDefinition($plugin_id . '.field');
       }
       // Checks to see if the target_object is an entity.
-      if (is_subclass_of($target) == 'Drupal\Core\Entity' || is_subclass_of($target, 'Drupal\Core\Entity\EntityInterface')) {
+      if (get_class($target) == 'Drupal\Core\Entity' || is_subclass_of($target, 'Drupal\Core\Entity\EntityInterface')) {
         $definition = $this->discovery->getDefinition($plugin_id . '.entity');
       }
     }
diff -u b/core/lib/Drupal/Core/Validation/Constraint/EntityTypeConstraint.php b/core/lib/Drupal/Core/Validation/Constraint/EntityTypeConstraint.php
--- b/core/lib/Drupal/Core/Validation/Constraint/EntityTypeConstraint.php
+++ b/core/lib/Drupal/Core/Validation/Constraint/EntityTypeConstraint.php
@@ -15,8 +15,8 @@
  *
  * @Plugin(
  *   id = "entity type",
- *   label = @Translation("Entity type constraint"),
- *   message = @Translation("The value of %label is not a valid entity type."),
+ *   label = @Translation("Entity type constraint", context = "Validation"),
+ *   message = @Translation("The value of %label is not a valid entity type.", context = "Validation"),
  *   type = "field"
  * )
  */
@@ -29,6 +29,17 @@
-   *   TRUE if there's a value, FALSE if the value is empty.
+   *   TRUE if value is a known entity type, FALSE if the value is empty.
    */
   public function validate($value) {
-     return TRUE;
+     return array_key_exists($value, entity_get_info());
   }
-}
+
+  /**
+   * Convert the constraint to form API structure.
+   *
+   * @return array
+   *   Array usable by the form API.
+   */
+  public function convertToFormAPI() {
+    return array(
+      '#choose' => array_keys(entity_get_info()),
+    );
+  }}
diff -u b/core/lib/Drupal/Core/Validation/Constraint/EqualFieldsConstraint.php b/core/lib/Drupal/Core/Validation/Constraint/EqualFieldsConstraint.php
--- b/core/lib/Drupal/Core/Validation/Constraint/EqualFieldsConstraint.php
+++ b/core/lib/Drupal/Core/Validation/Constraint/EqualFieldsConstraint.php
@@ -15,8 +15,8 @@
  *
  * @Plugin(
  *   id = "equal.field",
- *   label = @Translation("Equal fields"),
- *   message = @Translation("The value of %field1 is different from %field2."),
+ *   label = @Translation("Equal fields constraint", context = "Validation"),
+ *   message = @Translation("The value of %field1 is different from %field2.", context = "Validation"),
  *   type = "entity"
  * )
  */
@@ -30,7 +30,22 @@
    */
   public function validate($value) {
-    $this->addMessageArguments('%field1', $value->get($this->settings[0])->getName());
-    $this->addMessageArguments('%field2', $value->get($this->settings[1])->getName());
-    return $value->get($this->settings[0])->getValue() == $value->get($this->settings[1])->getValue();
+    $this->addMessageArguments('%field1', $value->get($this->properties['field1'])->getName());
+    $this->addMessageArguments('%field2', $value->get($this->properties['field2'])->getName());
+    return $value->get($this->properties['field1'])->getValue() == $value->get($this->properties['field2'])->getValue();
+  }
+
+  /**
+   * Convert the constraint to form API structure.
+   *
+   * @return array
+   *   Array usable by the form API.
+   */
+  public function convertToFormAPI() {
+    return array(
+      '#equal' => array(
+        $this->properties['field1'],
+        $this->properties['field2'],
+      ),
+    );
   }
 }
diff -u b/core/lib/Drupal/Core/Validation/Constraint/MinValueConstraint.php b/core/lib/Drupal/Core/Validation/Constraint/MinValueConstraint.php
--- b/core/lib/Drupal/Core/Validation/Constraint/MinValueConstraint.php
+++ b/core/lib/Drupal/Core/Validation/Constraint/MinValueConstraint.php
@@ -15,9 +15,12 @@
  *
  * @Plugin(
  *   id = "min",
- *   label = @Translation("MinValue"),
- *   message = @Translation("The value of %label has to be greater than %min."),
- *   type = "typedData"
+ *   label = @Translation("Min value constraint", context = "Validation"),
+ *   message = @Translation("The value of %label has to be greater than %min.", context = "Validation"),
+ *   type = "typedData",
+ *   properties = {
+ *     "min" = 0
+ *   }
  * )
  */
 class MinValueConstraint extends Constraint {
@@ -29,8 +32,8 @@
    *   TRUE if the value is greater or equal, FALSE if the value is smaller.
    */
   public function validate($value) {
-    $this->addMessageArguments('%min', current($this->settings));
-    return $value >= current($this->settings);
+    $this->addMessageArguments('%min', $this->properties['min']);
+    return $value >= $this->properties['min'];
   }
 
   /**
@@ -42,7 +45,7 @@
   public function convertToFormAPI() {
     return array(
       '#type' => 'number',
-      '#min' => current($this->settings),
+      '#min' => $this->properties['min'],
     );
   }
 }
diff -u b/core/lib/Drupal/Core/Validation/Constraint/MinValueIntegerConstraint.php b/core/lib/Drupal/Core/Validation/Constraint/MinValueIntegerConstraint.php
--- b/core/lib/Drupal/Core/Validation/Constraint/MinValueIntegerConstraint.php
+++ b/core/lib/Drupal/Core/Validation/Constraint/MinValueIntegerConstraint.php
@@ -15,9 +15,12 @@
  *
  * @Plugin(
  *   id = "min.integer",
- *   label = @Translation("MinValue"),
- *   message = @Translation("The value of %label has to be greater than %min."),
- *   type = "typedData"
+ *   label = @Translation("Min integer constraint", context = "Validation"),
+ *   message = @Translation("The value of %label has to be greater than %min.", context = "Validation"),
+ *   type = "typedData",
+ *   properties = {
+ *     "min" = 0
+ *   }
  * )
  */
 class MinValueIntegerConstraint extends MinValueConstraint {
@@ -30,6 +33,19 @@
    */
   public function validate($value) {
-    $this->addMessageArguments('%min', current($this->settings));
-    return $value >= current($this->settings);
+    $this->addMessageArguments('%min', $this->properties['min']);
+    return $value >= $this->properties['min'];
+  }
+
+  /**
+   * Convert the constraint to form API structure.
+   *
+   * @return array
+   *   Array usable by the form API.
+   */
+  public function convertToFormAPI() {
+    return array(
+      '#type' => 'number',
+      '#min' => $this->properties['min'],
+    );
   }
 }
diff -u b/core/lib/Drupal/Core/Validation/Constraint/NotNullConstraint.php b/core/lib/Drupal/Core/Validation/Constraint/NotNullConstraint.php
--- b/core/lib/Drupal/Core/Validation/Constraint/NotNullConstraint.php
+++ b/core/lib/Drupal/Core/Validation/Constraint/NotNullConstraint.php
@@ -15,8 +15,8 @@
  *
  * @Plugin(
  *   id = "notnull",
- *   label = @Translation("NotNull"),
- *   message = @Translation("The value of %label can not be null."),
+ *   label = @Translation("Not null constraint", context = "Validation"),
+ *   message = @Translation("The value of %label can not be null.", context = "Validation"),
  *   type = "typedData"
  * )
  */
@@ -33,2 +33,13 @@
   }
+
+  /**
+   * Convert the constraint to form API structure.
+   *
+   * @return array
+   *   Array usable by the form API.
+   */
+  public function convertToFormAPI() {
+    // @todo: implementqtion needed.
+    return array('#notnull' => TRUE);
+  }
 }
diff -u b/core/lib/Drupal/Core/Validation/Constraint/RequiredConstraint.php b/core/lib/Drupal/Core/Validation/Constraint/RequiredConstraint.php
--- b/core/lib/Drupal/Core/Validation/Constraint/RequiredConstraint.php
+++ b/core/lib/Drupal/Core/Validation/Constraint/RequiredConstraint.php
@@ -15,8 +15,8 @@
  *
  * @Plugin(
  *   id = "required",
- *   label = @Translation("Required"),
- *   message = @Translation("The value of %label is required."),
+ *   label = @Translation("Required constraint", context = "Validation"),
+ *   message = @Translation("The value of %label is required.", context = "Validation"),
  *   type = "typedData"
  * )
  */
diff -u b/core/lib/Drupal/Core/Validation/Constraint/RequiredFieldConstraint.php b/core/lib/Drupal/Core/Validation/Constraint/RequiredFieldConstraint.php
--- b/core/lib/Drupal/Core/Validation/Constraint/RequiredFieldConstraint.php
+++ b/core/lib/Drupal/Core/Validation/Constraint/RequiredFieldConstraint.php
@@ -15,8 +15,8 @@
  *
  * @Plugin(
  *   id = "required.field",
- *   label = @Translation("Required"),
- *   message = @Translation("The value of %label is required."),
+ *   label = @Translation("Required field constraint", context = "Validation"),
+ *   message = @Translation("The value of %label is required.", context = "Validation"),
  *   type = "field"
  * )
  */
diff -u b/core/modules/system/lib/Drupal/system/Tests/Validation/ValidationConstraintTest.php b/core/modules/system/lib/Drupal/system/Tests/Validation/ValidationConstraintTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/Validation/ValidationConstraintTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Validation/ValidationConstraintTest.php
@@ -50,2 +50,22 @@
   }
+
+  /**
+   * Tests MinValueConstraint
+   */
+  public function testMinValueConstraint() {
+    // Test using the default value for min.
+    $constraint = new Constraint\MinValueConstraint(array('properties' => array('min' => 0)), array());
+    $this->assertTrue($constraint->validate(5));
+    $this->assertTrue($constraint->validate(0));
+    $this->assertTrue($constraint->validate(''));
+    $this->assertFalse($constraint->validate(-2));
+
+    // Test using a custom value for min.
+    $constraint = new Constraint\MinValueConstraint(array('properties' => array('min' => 0)), array('min' => 5));
+    $this->assertTrue($constraint->validate(5));
+    $this->assertFalse($constraint->validate(4));
+    $this->assertFalse($constraint->validate(0));
+    $this->assertFalse($constraint->validate(-10));
+    $this->assertFalse($constraint->validate(''));
+  }
 }
only in patch2:
unchanged:
--- /dev/null
+++ b/core/lib/Drupal/Core/Validation/README.txt
@@ -0,0 +1,123 @@
+Rough technical overview of how it is implemented right now
+
+Constraints can be defined on 3 levels:
+entity
+ - field [0..n]
+   - typedData [1..n]
+ - property [1..n]
+   - typedData [1..n]
+
+FormController->build() calls entity->getConstraintsObjects()
+  - gets a list of constraints keyed by field/property name
+
+Each constraint has a method convertToFormAPI() which returns an array containing keyed data compatibnle with the form API
+ex:
+return array(
+  '#type' => 'number',
+  '#min' => current($this->settings),
+);
+
+FormController->validate() calls $entity->validate
+  this loops over all fields and typedData, calling validate() on each
+  validate() will get the constraints and validates the value/field
+  all violations are collected and returned to the parent
+  FormController uses set_form_error for each violation
+
+Constraints uses the Plugin system for it's definition
+ex:
+ * @Plugin(
+ *   id = "min",
+ *   label = @Translation("MinValue"),
+ *   message = @Translation("The value of %label has to be greater than %min.")
+
+The message is added to the plugin system so it gets detected by l.d.o.
+The Constraint base class has a method addMessageArguments so it can prefill some placeholders
+ex:
+$this->addMessageArguments('%min', current($this->settings));
+
+The FormController calls uses getMessage, getMessageArguments, getObjectLabel to build the (translated) message.
+ex:
+form_set_error('error', 
+  t($violation->getMessage(), 
+  $violation->getMessageArguments() + array('%label' => $violation->getObjectLabel()))
+);
+
+Constraints:
+Indented to show parent - child relationship.
+The factory used to choose the most appropriate Constraint accepts an object as parameter, this is either a TypedData, a Field or an Entity. The factory looks at the type of the object ('integer, 'field', ...) and uses the most specific class it can find.
+
+Constraint.php
+  EntityTypeConstraint.php --> array('#choose' => array_keys(entity_get_info()))
+  EqualFieldsConstraint.php --> array('#equal' => array($this->settings[0], $this->settings[1]))
+  MinValueConstraint.php --> array('#type' => 'number', '#min' => current($this->settings))
+    MinValueIntegerConstraint.php --> array('#type' => 'number', '#min' => current($this->settings))
+  NotNullConstraint.php --> array('#notnull' => TRUE)
+  RequiredConstraint.php --> array('#required' => TRUE)
+    RequiredFieldConstraint.php --> inherited
+
+Connect the dots: Entity to FormAPI
+entity -> $form['#constraints']
+  - field -> $form['field_name']['#constraints']
+  - typedata -> $form['field_name'][$delta]['#constraints']
+
+Form key always has to match the field_name, or use #bound_to
+FormController adds data (Constraint) to the form elements, in phase 1 it also adds the FormAPI structure so it can run side by side.
+AfterBuild converts Constraints to FormAPI structure, in phase 1 some FormAPI elements are supported as well.
+
+@TODO: getConstraints on each field has to return full objects
+@TODO: add afterBuild method to FormController to handle the conversion
+
+
+
+Follow up issues:
+Add support for validation on forms not tight to an entity.
+
+Add clientside validation support, see [#1797438]
+  add it in an afterBuild phase, so form_alter still works.
+  use clientside_validation, but allow rule definition on the constraint.
+
+Add constraint selection on instance_field_settings_form
+  which constraints do we need to show: structured tree containing field_type - widget_type
+  some constraint cannot be removed (like IntegerConstraint on an Integer text field)
+
+List all field types and subtypes (like integer select list) and
+  provide all constraints
+  organize the constraints in a tree (inheritance) structure
+
+How to handle conflicts (like max < min)?
+
+Do we need weight/importance/priority on Constraints?
+
+Decide if we still support old '#required'
+  example before
+    $form['name'] = array(
+      '#type' => 'textfield',
+      '#title' => t('Name'),
+      '#default_value' => 'default',
+      '#size' => 60,
+      '#weight' => -10,
+      '#maxlength' => 128,
+      '#required' => TRUE,
+    );
+  example after
+    $form['name'] = array(
+      '#type' => 'textfield',
+      '#title' => t('Name'),
+      '#default_value' => 'default',
+      '#size' => 60,
+      '#weight' => -10,
+      '#constraints' => array(
+        new MaxLengthConstraint(array(
+          'message' => 'or use default one from constraint',
+          'maxlength' => 128,
+        )),
+        new RequiredFieldConstraint(array(
+          'message' => 'or use default one from constraint',
+        )),
+      ),
+    );
+
+Validation phase has to pickup the custom messages
+
+
+