The problem is that I don't use Zend_Form, so I can't use the built in capabilities. I usually look for three things when setting a validator: - its path (usually in library/path/validators/MyValidator.php) - its class name (tried Validators_MyValidator) and - in application.ini, the autoloaderNamespaces[] = "Validators" I've tried different settings/namings/layouts to no avail. When using a validator in my Zend_Validate_Input, an exception occurs, saying that it couldn't find my plugin/validator, the classes being searched only in Zend/Validate with the Zend_Validate prefix (I didn't try this prefix, though). Wasn't there a setting in app.ini that added a prefix and a path?