High incidence and low survival rates of many epithelial cancers remain beyond the control of established preventive and therapeutic modalities. Chemoprevention is a new approach under study that involves the intervention within the premalignant process with specific chemical agents to reverse carcinogenesis and prevent the development of invasive cancer. The two biologic concepts that underlie this research are multistep carcinogenesis and field carcinogenesis. Major clinical issues include trial design and drug development in head and neck, lung, and breast cancer chemoprevention. Within the area of trial design, intermediate end point biomarkers will become very important for providing biologic insights in the short term and greater trial efficiencies in the long term. Drugs that are under the strongest investigation include retinoids and beta-carotene in the head and neck and lung, calcium in the colon, and tamoxifen in the breast. This new field has the potential to make an important contribution toward increasing our control over many deadly epithelial cancers Cancer 1993; 72:984-90.