Attention-deficit/hyperactivity disorder (ADHD) is a chronic, lifelong disorder with childhood-onset, which seriously impairs the affected adults in a variety of daily living functions like educational and occupational functioning, partnership and parenting. ADHD is associated with a high percentage of comorbid psychiatric disorders in every lifespan. In adulthood between 65-89% of all patients with ADHD suffer from one or more additional psychiatric disorders, above all mood and anxiety disorders, substance use disorders and personality disorders, which complicates the clinical picture in terms of diagnostics, treatment and outcome issues. The present overview provides information of comorbid psychiatric disorders in adults with ADHD, underlying associations and clinical implications.