For many years, discoveries about the genetic determinants of cancer appeared to be having only minor effects on efforts to control the disease in the clinic. Following advances made over the past decade, however, a description of cancer in molecular terms seems increasingly likely to improve the ways in which human cancers are detected, classified, monitored, and (especially) treated. Achieving the medical promise of this new era in cancer research will require a deeper understanding of the biology of cancer and imaginative application of new knowledge in the clinic, as well as political, social, and cultural changes.