Publisher Summary This chapter describes the general processes and concepts involved in the preclinical development of agents, with specific examples taken from the field of oncology. Preclinical development encompasses all of the activities that must take place before a new chemical entity can be administered to humans. Preliminary in vitro studies of a new pharmacologic agent are closely related to drug discovery and screening efforts. In vitro assessment of new drugs can be performed in intact cell lines or cell-free systems. These types of tests may form the basis for high-throughput screening assays employed in large-scale drug discovery or combinatorial chemistry efforts designed to identify new agents with specific molecular properties. Models resembling human-disease states may arise spontaneously in animals at a certain age or period of development or they may be induced by invasive interventions, such as treatments with drugs, chemical toxins, or radiation.