Attention-deficit/hyperactivity disorder (ADHD) is a prevalent disorder in adult psychiatry, particularly in out-patient settings. There are no objective, laboratory-based tests that can establish this diagnosis. Present diagnostic criteria for ADHD are formulated primarily according to behavior in childhood, based on age inappropriate and impairing levels of hyperactivity, impulsivity and inattention. Other symptoms, such as mood instability and frustration intolerance, are not included in current criteria for ADHD, but are very prevalent in this patient group. ADHD is often comorbid with alcohol and substance abuse and other psychiatric disorders, in particular anxiety and personality disorders. Thus, the diagnostic assessment should both include a comprehensive clinical interview, rating scales for past and present symptoms and collateral information from multiple informants, as well as assessment of a broader spectrum of psychiatric and somatic conditions. As ADHD is associated with changes in brain function mediating different aspects of neuropsychological functions, assessment of those functions is important to understand the symptom patterns and to develop targeted treatment programs. Some topics for further research and for future developments of diagnostic criteria and tools are highlighted.