Despite the fact that medication adherence has been extensively described in the literature over the last several decades, a quote by Becker and Maiman from over 35 years ago best captures the current state of our understanding: “Patient compliance[sic adherence] has become the best documented, but least understood, health behavior.” Future research is greatly needed to identify and translate safe and effective interventions into routine clinical practice to improve adherence. Only then can we begin to make significant improvements to the medication use process and, in turn, the health of older adults.