This chapter reviews sex differences in ADHD, focusing on differences in prevalence, comorbidity, and impairment, and discusses potential mechanisms underlying these differences. ADHD is more common in males than females (sex ratio ~3:1). Males with ADHD show greater comorbidity with comorbid externalizing (conduct) problems, while females with ADHD show internalizing problems. Females with ADHD may experience greater subjective impairment than males with ADHD. Referral and diagnostic issues, relating to sex-specific display of ADHD symptoms (more overt and disruptive in males, more subtle in females), underdiagnosis, or misdiagnosis in girls, as well as biases due to informant source, likely contribute to sex differences in ADHD. Potential biological mechanisms include endocrine factors (testosterone, glucocorticoids, and hypothalamic–pituitary–adrenal axis activation differences), aetiological sex differences (sex-chromosome genes), sex differences in neurocognitive functioning, and differences in brain structure and function. The chapter provides an outlook for future research and clinical implications.