Attention-deficit/hyperactivity disorder (ADHD) is one of the most common psychiatric disorders in childhood, affecting 5-7% of school-aged children and often persisting into adulthood. ADHD also presents with substantial heterogeneity, and there is rapidly accumulating evidence on the nature and importance of sluggish cognitive tempo (SCT) in children with and without ADHD. This seminar will provide up-to-date information on leading theories of ADHD, in addition to an overview of its presentations, co-occurring conditions, and associated impairments across the life span. Evidence-based assessment and behavioral treatments will be covered. The seminar will also cover the latest scientific evidence on SCT, including its history as related to and distinct from ADHD and other mental health conditions, currently available measures to guide assessment, impact on major domains of daily life functioning, and possible treatment approaches.