The rapid advances that have been made in molecular biology and molecular methods have led, and continue to lead, to the development of sensitive and specific diagnostic tests, which hold the promise of substantially strengthening our ability to diagnose, treat and control many of the major infectious diseases in developing countries. It is imperative that these new diagnostics are rigorously and properly evaluated in the situations in which they will be deployed in disease control before they are released for general use. A poorly performing diagnostic might not only waste resources but might also impede disease control. The basic procedures described in this booklet for designing and conducting diagnostic evaluations provide an outline for ensuring the proper evaluation of new diagnostics in laboratory and field trials.