Attention-deficit hyperactivity disorder (ADHD) is recognized to exist in males and females although 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 article provides a review of gender differences noted across the lifespan. Males and females with ADHD are more similar than different, and generally ADHD profiles 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; rates of depression and anxiety may be higher, and physical aggression and other externalizing behaviors lower in girls and women with ADHD. Men with ADHD seem to be incarcerated more often than women with ADHD. However, many studies suffer from small sample sizes, referral biases, differences in diagnostic procedures, and possible rater influences. Treatments are reviewed and discussed with reference to the reported gender differences in functioning and the global deficits noted in all samples. The data available so far suggest that treatments are likely to be equally effective in males and females. However, referral bias is a problem, in that females with ADHD are less likely to be referred for treatment than males with ADHD. Future research should include equal representation of both sexes in samples such that sex by treatment analyses can be routinely conducted.