Attention-deficit/hyperactivity disorder (ADHD) rarely occurs in isolation and is frequently associated with a range of psychiatric and medical comorbidities. Common psychiatric comorbid conditions include depressive disorders, anxiety disorders, substance use disorders, and learning disabilities, while frequently associated medical conditions include sleep disorders, allergic conditions, and migraine headaches. The presence of overlapping symptoms may complicate diagnostic accuracy, resulting in misdiagnosis, delayed diagnosis, or incomplete recognition of co-occurring conditions. Individuals may be incorrectly diagnosed with an alternative disorder when ADHD remains unrecognized, misdiagnosed with ADHD when another condition is primary, or present with both ADHD and additional disorders concurrently. This presentation will review key considerations in the assessment of comorbid conditions and differential diagnosis in individuals with suspected or confirmed ADHD. Emphasis will be placed on identifying distinguishing clinical features, understanding symptom overlap, and recognizing patterns suggestive of secondary conditions arising from untreated ADHD. Additionally, the bidirectional impact of ADHD and comorbid disorders on clinical presentation, functional impairment, and treatment response will be examined. Clinical case examples will be used to illustrate diagnostic challenges and inform treatment planning.