The multistep process of carcinogenesis, which can take many years, provides many opportunities for intervention to inhibit disease progression. Effective chemoprevention agents may reduce the risk of cancer by inhibiting the initiation stage of carcinoma through induction of apoptosis or DNA repair in cells harboring mutations, or they may act to prevent promotion of tumor growth. Similarly, chemoprevention may entail blocking cancer progression to an invasive phenotype. Over the past decade, in vitro, preclinical, and clinical data have supported the hypothesis that cyclooxygenase (COX)-2 plays a central role in oncogenesis and that treatment with COX-2 inhibitors offers an effective chemoprevention strategy, as exemplified by the activity of celecoxib (Celebrex) in familial adenomatous polyposis. These COX-2 data have contributed to initiation of clinical trials testing COX-2 inhibitors for the chemoprevention of a wide variety of cancers that overexpress COX-2.