Any effective screening program must satisfy 2 criteria: 1) the test must identify clinically significant disease earlier than its clinical presentation, and 2) a treatment must be available that will alter the natural history of the disease. The controversy surrounding PSA testing that has raged since 1991 centers on these 2 points. Screening and treatment trials published during the past 3 decades have provided critical insights into our understanding of the natural history of PSA identified cancers and the impact of treatment. This in turn raises questions concerning the mechanism of prostate cancer mortality reduction. This essay reflects on the mechanisms of disease progression and the implications for future screening and treatment efforts.