Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder that often persists into adulthood. Adults may first receive a diagnosis in primary care, sometimes after previously unrecognized childhood symptoms. Reports of de novo “adult-onset ADHD” remain controversial and, when scrutinized, are most often explained by previously subthreshold childhood symptoms, alternative psychiatric or medical causes, or measurement error. If an adult presents with symptoms of ADHD but onset cannot be substantiated before age 12 years, providers should systematically rule out mimics—which include mood and anxiety disorders, substance use disorder, sleep disorders, traumatic brain injury, thyroid disorders, and medication side effects—before labeling the presentation as adult-onset ADHD. This activity outlines the epidemiology, pathophysiology, clinical features, diagnostic strategies, and evidence-based treatments for ADHD presentations in adults. Primary care clinicians play a key role in recognizing symptoms, initiating evaluation, and implementing multimodal treatment plans to improve patient outcomes.