Suboptimal adherence to treatment regimens is a major obstacle to treatment efficacy and positive outcomes for patients. While poor adherence is common across a variety of chronic conditions, an area which presents unique challenges to clinicians and researchers is non-adherence among pediatric populations. These challenges are well illustrated by the management of attention-deficit/hyperactivity disorder (ADHD), a pervasive pediatric psychiatric condition. The average rates of non-adherence in children and adults ranged between 15 and 87%. Factors predicting increased adherence/persistence included the use of long-acting formulations, younger age, Caucasian background, family structure and the presence and treatment of comorbidities. Decreased adherence/persistence were predicted by multiple daily dosing, family history of ADHD, experiences of adverse effects, stigma and treatment inefficacy. The broad range of non-adherence rates identified reflects the complexities of adherence research in ADHD, and highlights the need for better standardization of adherence/persistence definitions and measurement approaches.