This chapter has outlined some principles of tumor growth, test characteristics, and the evaluation of screening technologies. We have emphasized that test specificity is the critical parameter in the evaluation of technologies because it is the healthy people who will suffer the most from the adverse effects of screening. We have also emphasized that the efficacy of a test is best evaluated by examining mortality reductions in comparable populations. The purpose has been to assist clinicians with their interpretation of the literature. Busy clinicians may not always have the time or inclination to do this themselves. In those cases they need to examine how organizations who make recommendations are coming to their conclusions because it is physicians, not organizations that will do the screening. In particular, it is important to ask the following: (1) Were criteria followed to justify the recommendations being made? (2) If so, what were they, and can the organization demonstrate that they are being met? (3) What perspectives and biases do the organizations bring to the judgments they inevitably have to make? (4) Do you share those perspectives? and (5) When the recommendation is adopted, can you guarantee that it will "first do no harm."