With the rise in availability and increasing use of oral anticancer agents, concerns about adherence to prescribed regimens will become an increasingly important issue in oncology. Few published studies have focused on adherence to oral antineoplastic therapy, in part because the vast majority of chemotherapy is delivered intravenously in physicians' offices or hospitals. In this article, we review current knowledge of adherence behavior with regard to oral medications in general, including factors associated with adherence and methods for measuring adherence. We also review published studies of adherence to oral antineoplastic agents in adult and pediatric populations and adherence issues in cancer prevention. The available evidence reveals that patient adherence to oral chemotherapy recommendations is variable and not easily predicted. Adherence rates ranging from less than 20% to 100% have been reported, and certain populations, such as adolescents, pose particular challenges. Future efforts should focus on improving measurement and prediction of adherence and on developing interventions to improve adherence for both patients in clinical trials and patients being treated outside of the research setting. Assessment of adherence among individuals with cancer and implementation of interventions in situations of poor adherence should improve clinical outcomes.