Significant progress has been made in recent years in developing more effective and better tolerated means to prevent nausea and vomiting induced by cancer chemotherapy. The most significant development has been the introduction of a new class of antiemetic agents, the selective antagonists of the type 3 serotonin receptor. With the new antiemetic therapeutic options and their attendant higher costs has come a need to define evidence-based guidelines to assist in their judicious and cost-effective use. A number of predictive factors for antiemetic risk have been defined. Some of these factors relate to the patient population (age, gender, history of ethanol consumption, and prior experience with chemotherapy), and some relate to the treatments administered. Clearly, the most important of all these factors in predicting risk of emesis is the intrinsic emetogenicity of the chemotherapy. Although an "ideal" emetogenic classification schema for chemotherapy has yet to be realized, recent developments in this area have allowed a more precise estimation of emetogenic risks and have provided antiemetic guideline groups with a useful foundation on which to base their treatment recommendations.