The retinoids are a class of compounds structurally related to Vitamin A. In the last 30 years, more than 2,500 retinoids have been synthesized and biologically tested, with the objective of identifying those with an enhanced therapeutic ratio. The majority have been studied for their cancer-preventive activity in experimental models, and some in clinical trials in humans. In March 1999, a working group of international experts met in Lyon to consider the existing evidence on the cancer-preventive activity of nine retinoids judged to have sufficient human and/or animal data to permit evaluation. This handbook details the evidence considered, and summarizes the evaluation made be the working group.