Experienced cancer researchers from pharmaceutical companies, government laboratories, and academia comprehensively review and describe the arduous process of cancer drug discovery and approval. They focus on using preclinical in vivo and in vitro methods to identify molecules of interest, detailing the targets and criteria for success in each type of testing and defining the value of the information obtained from the various tests. They also define each stage of clinical testing, explain the criteria for success, and outline the requirements for FDA approval. A companion volume by the same editor (Cancer Therapeutics: Experimental and Clinical Agents) reviews existing anticancer drugs and potential anticancer therapies. These two volumes in the Cancer Drug Discovery and Development series reveal how and why molecules become anticancer drugs and thus offer a blueprint for the present and the future of the field.