Phase I trials are the first step in the study of novel therapeutic approaches. They aim to identify the recommended dose, schedule and pharmacologic behavior of new drugs, and to describe the adverse effects of treatment. This practical guide discusses the design, conduct, analysis and ethical aspects of Phase I cancer trials.About the Author: Elizabeth Eisenhauer was appointed to the Board of Directors of the National Cancer Institute of Canada in 2002, and will become President in June 2006. Through her work with the IND program and NCIC CTG, she has co-ordinated over 170 Phase I, II and III trials carried out across Canada, the US andEurope. Marc Buyse founded the International Drug Development Institute (IDDI) in 1991. In addition to his current posts, he has previously been President of the International Society for Clinical Biostatistics and a Board Member of the Society for Clinical Trials. He has an editorial role in thefollowing journals: Biometrical Journal, Clinical Trials, Journal of Clinical Oncology, and Statistical Methods in Medical Research.