Adherence, often referred to as compliance, is defined as the extent to which a person's health-related behaviors coincide with agreed recommendations from a health care provider.[1] Difficulties with adherence have been described since the time of Hippocrates. Nonadherence with drug therapy may reflect the failure to fill prescribed medications, delayed or forgotten doses, incorrect amounts of medication or improper dosing intervals, and failure to complete full courses of therapy. For children, the problems surrounding drug adherence are unique because the adherence of the parent and the child must be considered. This chapter focuses on adherence with medication use to illustrate the extent and consequences of nonadherence in pediatrics, determinants of nonadherence, and strategies to improve adherence within a pediatric population.