Attention-Deficit/Hyperactivity Disorder (ADHD), although considered a childhood-onset neurodevelopmental condition, is nevertheless a frequent and disabling condition in adults. A proportion of such patients are not diagnosed during childhood or adolescence, as diagnosis of the syndrome is rather complex, especially when other psychiatric, neurological or other neurodevelopmental conditions are also associated, yet comorbidities and consequences of ADHD are frequently observed in adults and older populations. As ADHD patients present to memory clinics with attentional and executive disorders, neuropsychological examinations of undiagnosed ADHD patients may reveal atypical cognitive profiles that can complicate the usual diagnostic procedure and increase the risk of delayed diagnosis or misdiagnosis. Thus, explorations of cognitive and/or behavioral disorders in adult populations should systematically screen for this neurodevelopmental condition. Accurate diagnosis could lead to non-pharmaceutical and/or pharmaceutical treatments to improve symptoms and quality of life for adult ADHD patients.