The validation of a test method is the process by which the relevance and reliability of the method are assessed for a particular purpose. It is an essential stage in the evolution of the method from its development to its acceptance and application for regulatory purposes. The principles according to which alternative tests should be validated have been agreed at an international level, although the actual process by which the validation process is conducted varies between different validation authorities. This paper summarises the principles of alternative test development and validation, and describes how the principles have been applied to the validation of in vitro tests by ECVAM.