A number of evidence-based treatments are available for attention-deficit/hyperactivity disorder (ADHD), including pharmacological, psychosocial, or a combination of the two treatments. For a significant number of children diagnosed with ADHD, however, these treatments are not utilized or adhered to for the recommended time period. Given that adherence to treatment regimens is necessary for reducing the symptoms of ADHD, it is crucial to develop a comprehensive understanding of why adherence rates are so low. The current review examines the literature to date that has directly explored utilization and adherence issues related to the treatment of ADHD in order to identify the key barriers to treatment. This review focused on four main factors that could account for the poor rates of treatment utilization and adherence: personal characteristics (socio-demographic characteristics and diagnostic issues), structural barriers, barriers related to the perception of ADHD, and barriers related to perceptions of treatment for ADHD. This review included 63 papers and covered a variety of barriers to treatment that have been found in research to have an impact on treatment adherence. Based on this review, we conclude that there are complex and interactive relationships among a variety of factors that influence treatment utilization and adherence. Four main gaps in the literature were identified: (1) there is limited information about barriers to psychosocial interventions, compared to pharmacological interventions; (2) there is a limited variety of research methodology being utilized; (3) treatment barrier knowledge is mostly from parents' perspectives; and (4) treatment utilization and treatment adherence are often studied jointly. Information from this review can help practitioners to identify potential barriers to their clients being adherent to treatment recommendations.