Chapter 10 introduces the reader to the methodology of measuring the accuracy of several medical tests that may be administered to a patient. Our main focus is on measuring the accuracy of a combination of two or more tests. For example, to diagnose type 2 diabetes, the patient is given a fasting blood glucose test, which is followed by an oral glucose tolerance test. What is the accuracy (true positive fraction [TPF] and false positive fraction [FPF]) of this combination of two tests? Or, in order to diagnose coronary artery disease, the subject’s history of chest pain (CPH) is followed by an exercise stress test (EST). Still another example is for the diagnosis of prostate cancer, where a digital rectal exam is followed by measuring prostate-specific antigen (PSA). The reader is referred to Johnson and Sandmire [1] for a description of additional examples of multiple tests to diagnose a large number of diseases, including heart disease, diabetes, lung cancer, breast cancer, etc.