Recent studies suggest an association between attention-deficit/hyperactivity disorder (ADHD) and obesity. In this article, we systematically review and critically discuss evidence on the prevalence of ADHD in obese patients as well as the weight status of individuals with ADHD. Relevant articles were searched in PubMed, PsychInfo, and ISI Web of Science (January 1980 to June 2010). We found that current evidence indicates a high prevalence of ADHD in clinical samples of patients seeking treatment for their obesity. Moreover, available studies show that individuals with ADHD have higher-than-average body mass index z scores and/or a significantly higher prevalence of obesity compared with subjects without ADHD. Three mechanisms underlying the association between ADHD and obesity have been proposed: 1) it is possible that obesity and/or factors associated with it (such as sleep-disordered breathing) manifest as ADHD-like symptoms; 2) ADHD and obesity share common biological dysfunctions; and 3) ADHD contributes to obesity. With regards to the possible clinical implications, our findings suggest that it is noteworthy to screen for ADHD in patients with obesity and to look for abnormal eating behaviors as possible contributing factors of obesity in patients with ADHD. Based on preliminary findings, appropriate treatment of ADHD may improve the weight status of individuals with both obesity and ADHD.