Adherence to oral chemotherapy regimens maximizes their effectiveness and minimizes any potential toxicities. Factors specifically related to the treatment, patient, and health care provider may influence medication adherence. Treatment-related factors include the complexity of the regimen, the cost of therapy, the possibility of side effects, and the delay in treatment benefits. Meanwhile, patients may not have an adequate support system or an understanding of the need for the medication, and providers may not fully succeed in communicating the importance of adherence and the types of side effects that may occur. Nonadherence may lead to an increased risk of toxicity, decreased effectiveness, and increased utilization of health care resources. Although various methods for measuring adherence are available, self-reporting is the most widely used. Studies describing adherence in a broad range of cancers are reviewed. Treatment of chronic myeloid leukemia has been revolutionized by the development of oral tyrosine kinase inhibitors that are highly effective in managing the disease when taken consistently. However, nonadherence is relatively common and can lead to reduced rates of response and increased medical costs. Similar effects of nonadherence on outcome and cost have also been observed in patients with various other hematologic malignancies and solid tumors. Interventions to improve adherence to oral chemotherapy regimens include communication about the importance of adherence and the potential consequences of nonadherence, simplification of the patient's medication schedule (if possible), and inclusion of a caregiver or family member in the conversation. Written materials should always be provided to accompany verbal instructions. This review summarizes factors influencing medication adherence, impact of nonadherence on patient outcomes, methods for measuring adherence, previous studies of nonadherence in patients with cancer, common barriers to access, and interventions to improve adherence in the community setting.