In recent years prevention has become extremely important in the war against cancer. For many cancers, major risk factors are not amenable to change and, therefore, secondary prevention through screening and early detection is the major type of intervention. Furthermore, with the discovery of cancer genes and tumor markers, which make it easier than ever before to identify people with increased risk of developing certain types of cancers, the role of secondary prevention has assumed an even greater value. This paper reviews recent advances in secondary prevention of those cancers for which overall efficacy of screening to reduce mortality has been demonstrated but for which some residual controversies exist: breast, cervical, and colorectal cancers.