The rapid pace of research in the field of genetics has already yielded many benefits. The development of new genetic tests is one such example. Before there can be widespread uptake of these tests they need to be evaluated to confirm the benefits of their use. The authors review some of the key features of the evaluation of diagnostic tests focusing on analytical and clinical validity. Test properties such as sensitivity, specificity, likelihood ratios, positive and negative predictive values, and how they relate to molecular genetic testing are discussed. Associated issues such as the concepts of disease definition, imperfect reference standards, and false positives are also explored. The authors suggest possible approaches to addressing some of the problems identified.