The purpose of this document is to facilitate the proper application and interpretation of the GLP Principles for the organisation and management of in vitro studies, and to provide guidance for the appropriate application of the GLP Principles to in vitro studies, both for test facilities (management, QA, study director and personnel), and for national GLP compliance monitoring authorities. This Advisory Document intends to provide such additional interpretation of the Principles and guidance for their application to in vitro studies carried out for regulatory purposes. It is organised in such a way as to provide easy reference to the GLP Principles by following the sequence of the different parts of these GLP Principles.