This chapter reviews the literature on comorbidity, differential diagnosis, and treatment response of attention-deficit hyperactivity disorder (ADHD) and anxiety disorders in adults. Anxiety disorders include generalized anxiety disorders (GAD), post traumatic stress disorder (PTSD), panic disorder, obsessive-compulsive disorder, phobias, and social anxiety disorder. The chapter suggests that anxiety disorders and ADHD represent independent disorders, each of which contributes to increasing comorbidity with independent and additive contributions to morbidity. There is suggestive evidence that ADHD is anxiogenic and that anxiety exacerbates problems with working memory and attention; there is inconsistent evidence suggesting that anxiety in the presence of ADHD may act as a modest brake on disinhibition and impulsivity. The multimodal treatment of ADHD study (MTA) found that a combination of medication and psychological treatment offered some differential benefit over medication alone in comorbid ADHD with anxiety.