Attention-deficit/hyperactivity disorder (ADHD) has now been recognized to exist in both males and females, albeit the literature supports a higher prevalence in males. However, when girls are diagnosed with ADHD, they are more often diagnosed as predominantly inattentive than boys with ADHD. This paper provides a review of gender differences noted across the lifespan in terms of psychosocial functioning, cognitive abilities and psychiatric comorbidities. Males and females with ADHD are more similar than different, and generally symptoms of ADHD are not sex specific. Small gender differences have been found: adolescent girls with ADHD have lower self-efficacy and poorer coping strategies than adolescent boys with ADHD, but these differences tend to disappear by adulthood; rates of depression and anxiety may be higher (especially in adolescence) while physical aggression and other externalizing behaviors may be lower in girls and women with ADHD, although not all studies support these findings (e.g., non-referred samples show similar rates of coexisting psychiatric disorders between boys and girls with ADHD). However, many studies suffer from small sample sizes, referral biases, differences in diagnostic procedures and possible rater influences. Psychosocial treatments are reviewed and discussed with reference to the reported gender differences in functioning as well as the global deficits noted in all samples. Although the data available so far suggest that psychosocial treatments are likely to be equally effective in males and females, this conclusion is based more on the small number of gender differences noted in overall functioning and less on empirical research on treatment by sex effects and the moderating role of sex, an effect only investigated by the Multi-modal Treatment Study of ADHD group, to date. Future research should include equal representation of both sexes in samples such that treatment analyses by gender can be routinely conducted.