Despite improved breast cancer survival rates with the use of tamoxifen and aromatase inhibitors, patients remain at risk for cancer recurrence and mortality because of nonadherence to medication. The objective of this review was to identify factors associated with nonadherence among patients with breast cancer. Electronic databases were searched for studies, and potentially relevant studies were retrieved and assessed for eligibility. Potential factors associated with nonadherence were identified, and they included patient-related factors (e.g., patient beliefs and knowledge, fear of adverse effects, forgetfulness, smoking, age, race), therapy-related factors (e.g., duration, side effects, additional prescribed medications, treatment interfering with lifestyle), healthcare system factors (e.g., patient/provider relationships), socioeconomic factors (e.g., medication costs, burden, scheduling problems, religion, marital status), and disease-related factors (e.g., comorbidities, stage of breast cancer). Those findings highlight the need for development of interventions to promote long-term adherence in patients with breast cancer.