The purpose of this document is to provide guidance on issues related to the validation of new or updated test methods consistent with current approaches. This document provides a synopsis of the current state of test method validation, in what is a rapidly changing and evolving area. Guidance on the more general aspects of OECD Test Guideline development is provided in the OECD Guidance Document for the development of OECD Test Guidelines for the testing of chemicals. While the principles of validation as described in this document were written for biology-based tests, they may be applicable to other Test Methods.