During the last few years several clinical guidelines on attention-deficit hyperactivity disorder (ADHD) have been published by national and international medical societies. To systematically review and compare recommendations of selected ADHD guidelines, we performed a systematic search in online guideline databases and PubMed in order to retrieve guideline texts. Guidelines meeting inclusion criteria were reviewed and recommendations on assessment and treatment extracted. The AGREE instrument was used to assess methodological quality. Of the 26 guidelines identified, 13 were selected for further analysis: 11 guidelines deal with ADHD in childhood and adolescence and 5 guidelines cover transitional patients and/or ADHD in adults. The methodological quality of ADHD guidelines is moderate to good. They reflect similarities and differences of healthcare systems. Diagnosis throughout the lifespan is based on a detailed clinical history. There is greater agreement on evidence-based pharmacological treatment than on psychosocial interventions, reflecting the strength of evidence.