ADHD is increasingly being diagnosed in adulthood. The current criteria for diagnosis of ADHD (DSM-IV) have been developed and validated for children and are inappropriate for adults. However, since DSM-IV criteria are currently the main recognized diagnostic criteria in use, clinicians are left with little alternative. This review explores the issues that complicate the diagnosis of ADHD in adulthood. It outlines the limitations of the DSM-IV regarding the diagnosis of ADHD in adults, describes how DSM-V proposes to address some of these limitations and discusses the challenges of differential diagnosis and comorbidity. Suggestions for dealing with these challenges and future perspective for improving the diagnostic validity of ADHD in adults are provided.