Attention-defi cit/hyperactivity disorder (ADHD) is prevalent, chronic, and often comorbid with other disorders. This article reviews the literature on ADHD and comorbid major depressive disorder from childhood through adolescence and adulthood. We also report fi ndings from a study we conducted that examined the prevalence of ADHD and comorbid major depression in adults. Diagnostic history data were analyzed in 524 adults (308 men; 216 women), 383 of whom had a diagnosis of ADHD. Adults with ADHD were more likely to have current major depression than adults without ADHD. Adults with the combined subtype of ADHD were more likely to have past and current major depression than those with the inattentive and hyperactive impulsive subtypes. Men and women with ADHD were more likely to have current major depression than men and women without ADHD, but women in the inattentive and combined subtype groups were more likely to have had past major depression than women in the hyperactive impulsive subtype group. Implications for diagnosis and treatment are discussed.