Undiagnosed attention-deficit/hyperactivity disorder (ADHD) can have considerable lifetime impacts on the individual, affecting academic, employment, and social outcomes, making accurate diagnosis vital. However, the diagnosis of ADHD in adolescents and emerging adults is not a straightforward activity. Many other conditions and factors can cause individuals to experience or report symptoms that overlap with those of ADHD, and most of these mimicking conditions occur at a higher base rate than does ADHD itself. Given recent concerns raised regarding the possible overdiagnosis of ADHD in this population, this chapter offers research-informed guidelines to clinicians regarding how to determine whether a previously undiagnosed adolescent or adult actually meets diagnostic criteria for ADHD. Common diagnostic challenges are described, including the impacts of psychological conditions, social media, and secondary gain. The chapter authors conclude by providing a comprehensive assessment approach, illustrated with the help of relevant case examples and vignettes.