CONTEXT: The use of DNA- and RNA-based tests continues to grow for applications as varied as inherited disease, infectious disease, cancer, identity testing, human leukocyte antigen typing, and pharmacogenetics. Progress is driven in part by the huge growth in knowledge about the molecular basis of disease coupled with advancements in technologic capabilities. In addition to requirements for clinical utility, every molecular test also may have limitations that must be carefully considered before clinical implementation. Analytic and clinical performance characteristics as well as test limitations are established and documented through the process of test validation. OBJECTIVE: To describe the established principles of test validation, along with relevant regulations in the United States, in order to provide a rational approach to introducing molecular tests into the clinical laboratory. DATA SOURCES: PubMed review of published literature, published guidelines, and online information from national and international professional organizations. CONCLUSIONS: These resources and recommendations provide a framework for validating clinical tests.