Tamoxifen is an essential drug in treating hormone receptor-positive breast cancer and has been used successfully for the past three decades. More recently, aromatase inhibitors (AIs) have also shown great promise in reducing breast cancer recurrence. To receive the optimal benefits, patients need to take these drugs for a period of 5 years. Yet, despite the known positive patient outcomes associated with their use, adherence to both tamoxifen and AIs is substantially less than ideal. This article reviews the most recent literature reporting adherence rates for tamoxifen and AIs, as well as correlates of adherence. Factors that help to explain nonadherence are reviewed, including the side-effect profile, and approaches to intervention to enhance adherence are discussed.