Evaluation of selectivity is one of the most controversial aspects of method validation and application of methods to studies. The focus of selectivity testing should address the question: Above what level will interference significantly impact on study conclusions? Four key issues will be addressed: the statistical relevance of any selectivity test performed; a criterion for significant interference; experimental methods to establish selectivity; and criteria for acceptance. To ensure that compound integrity is maintained throughout the work-up process, statistically meaningful methods of stability evaluation which are associated with specific acceptance criteria are required. Suitable methods for evaluating stability of analyte and/or solutions of analyte, in process stability, processed sample stability, long term stability and freeze-thaw stability, as well as meaningful acceptance criteria, are presented.