diff --git a/calsocial/forms.py b/calsocial/forms.py
index 509c152..da72b2a 100644
--- a/calsocial/forms.py
+++ b/calsocial/forms.py
@@ -23,16 +23,78 @@ from flask_wtf import FlaskForm
 import pytz
 from wtforms import BooleanField, PasswordField, SelectField, StringField
 from wtforms.ext.dateutil.fields import DateTimeField
-from wtforms.validators import DataRequired, Email, ValidationError
+from wtforms.validators import DataRequired, Email, StopValidation, ValidationError
 from wtforms.widgets import TextArea
 
 
+class UsernameAvailable(object):  # pylint: disable=too-few-public-methods
+    """Checks if a username is available
+    """
+
+    def __init__(self, message=None):
+        self.message = message
+
+    def __call__(self, form, field):
+        from sqlalchemy.orm.exc import NoResultFound
+
+        from calsocial.models import User
+
+        # If there is no data, we don’t raise an error; it’s not the task of this validator to
+        # check the validity of the username
+        if not field.data:
+            return
+
+        try:
+            User.query.filter(User.username == field.data).one()
+        except NoResultFound:
+            return
+
+        if self.message is None:
+            message = field.gettext('This username is not available')
+        else:
+            message = self.message
+
+        field.errors[:] = []
+        raise StopValidation(message)
+
+
+class EmailAvailable(object):  # pylint: disable=too-few-public-methods
+    """Checks if an email address is available
+    """
+
+    def __init__(self, message=None):
+        self.message = message
+
+    def __call__(self, form, field):
+        from sqlalchemy.orm.exc import NoResultFound
+
+        from calsocial.models import User
+
+        # If there is no data, we don’t raise an error; it’s not the task of this validator to
+        # check the validity of the username
+        if not field.data:
+            return
+
+        try:
+            User.query.filter(User.email == field.data).one()
+        except NoResultFound:
+            return
+
+        if self.message is None:
+            message = field.gettext('This email address can not be used')
+        else:
+            message = self.message
+
+        field.errors[:] = []
+        raise StopValidation(message)
+
+
 class RegistrationForm(FlaskForm):
     """Registration form
     """
 
-    username = StringField(_('Username'), validators=[DataRequired()])
-    email = StringField(_('Email address'), validators=[Email()])
+    username = StringField(_('Username'), validators=[DataRequired(), UsernameAvailable()])
+    email = StringField(_('Email address'), validators=[Email(), EmailAvailable()])
     password = PasswordField(_('Password'), validators=[DataRequired()])
     password_retype = PasswordField(_('Password, once more'), validators=[DataRequired()])
 
diff --git a/calsocial/templates/registration.html b/calsocial/templates/registration.html
index cb1a69c..95f3977 100644
--- a/calsocial/templates/registration.html
+++ b/calsocial/templates/registration.html
@@ -2,6 +2,7 @@
 
 {% block content %}
 <form method="post">
+    {{ form.errors }}
     {{ form.hidden_tag() }}
 
     {{ form.username.errors }}