This guide is a practical companion based on consensus-based recommendations for co-occurring and differential diagnosis in ADHD assessments for school-aged children. It is designed to be used selectively, based on the presenting concerns of each client. These recommendations were developed through consensus (Delphi method) with psychologists from Australia, New Zealand, Canada, and the USA, and are intended for use by registered or licensed psychologists (or equivalent) with appropriate training in psychological assessment. They have not yet been piloted in clinical practice, and this guide is intended to support, not replace, a full clinical assessment. Psychologists should use these recommendations in conjunction with their professional judgement, relevant diagnostic manuals (DSM-5-TR, ICD-11), applicable clinical practice guidelines, and established ADHD practice guidelines.