The issue of medication noncompliance is becoming increasingly important in oncology as more cancer therapies are delivered orally. Medication adherence is difficult to assess and there is no gold standard of measurement. The act of measuring adherence can affect outcomes. Medication noncompliance is common, and is estimated to be 50% in treatment of chronic diseases. Studies have shown that women initiate adjuvant hormonal therapy for breast cancer 64% to 88% of the time when prescribed. Of those who initiate therapy, 50% to 80% are adherent for the prescribed duration, depending on the study. Patients noncompliant with adjuvant hormonal therapy for breast cancer have worse overall survival than their counterparts. Suboptimal treatment responses in chronic myeloid leukemia (CML) are also associated with medication noncompliance. Poor adherence can also affect clinical trial results, leading to inaccuracies of treatment efficacy. Barriers to compliance can occur on the individual, cultural, or system level. Examples of specific barriers are side effects, cost and access to medication, and individual health beliefs. Specific populations, including racial minorities, elderly patients, and very young patients, may be at higher risk for medication noncompliance. Strategies to improve compliance are multifactorial and include improvement of patient education, reduction of treatment side effects, interventions to alter behavior, and changes in public policy to improve financial barriers to treatment. Technology has been an effective tool in improving compliance in noncancer-related illness, and ongoing studies are evaluating its role in the oncology population.