Much of clinical research is aimed at assessing causality. However, clinical research can also address the value of new medical tests, which will ultimately be used for screening for risk factors, to diagnose a disease, or to assess prognosis. In order to be able to construct research questions and designs involving these concepts, one must have a working knowledge of this field. In other words, although traditional clinical research designs can be used to assess some of these questions, most of the studies assessing the value of diagnostic testing are more akin to descriptive observational designs, but with the twist that these designs are not aimed to assess causality, but are rather aimed at determining whether a diagnostic test will be useful in clinical practice. This chapter will introduce the various ways of assessing the accuracy of diagnostic tests, which will include discussions of sensitivity, specificity, predictive value, likelihood ratio, and receiver operator characteristic curves.