Nausea and vomiting induced by several cancer chemotherapy agents is often the most distressing side effect of treatment. The mechanisms are quite complex. The vomiting center in the reticular formation can be stimulated by either afferent Stimuli from the gastrointestinal tract or by the chemoreceptor trigger zone (CTZ). The latter is probably the primary site for emetic activity of most cancer chemotherapeutic agents and is accessible to drugs that do not cross the blood-brain barrier. It is quite possible that several agents have different receptors. The wide spectrum of antiemetics is in contrast to the often observed lack of effectiveness. The more successful trials have concentrated on agents, doses, schedules, or routes of administration that were not generally used prior to 1980. An effective study design has reduced methodological difficulties and reproducible data have been reported.