Attention deficit/hyperactivity disorder (ADHD) is estimated to affect 4-5% of adults. Impairment across multiple domains of daily living can be mild to serious. OROS methylphenidate (MPH) was evaluated in two large adult clinical trials, and in 2008 it was approved in the USA to treat ADHD in adults aged up to 65 years. Products approved for adult ADHD; chemistry, pharmacodynamics, pharmacokinetics, metabolism, efficacy, safety, tolerability, abuse liability, regulatory affairs, and relative merits and limitations associated with OROS MPH; and complexities associated with diagnosis and treatment of ADHD are discussed in this review. The reader will gain an introduction to the epidemiology, features and special issues regarding clinical management of ADHD in adults. Key features and development of OROS MPH are presented, and unique characteristics of the delivery system and their implications for clinical use of the drug are discussed. The OROS system delivers MPH in a highly consistent manner using osmotic pressure at a controlled rate over a 12-h duration of effect. Compared with immediate-release MPH, it provides a smoother pharmacokinetic curve, and it may show greater patient compliance, fewer missed doses and reduced abuse liability.