Attention-deficit/hyperactivity disorder (ADHD) is a chronic disease that is well accepted as a childhood condition. Despite increasing evidence of its clinical relevance in adults, it would appear that adult ADHD is underdiagnosed. This is particularly the case when comorbid with another mental disorder. Comorbidity across the life-span runs as high as 70% amongst adults diagnosed with ADHD. One of the most frequently occurring comorbidities in adult ADHD are substance use disorders (SUDs), which show a bi-directional relationship. ADHD is a risk factor for the development of later SUD to the extent that 9%-30% of adults with ADHD have a substance use problem. On the other hand, prevalence studies have shown that between 15% and 25% of patients with a SUD also have ADHD. The bi-directional relationship between ADHD and SUD can modify the clinical expression of symptoms, thus rendering difficult both correct diagnosis and appropriate treatment. ADHD is a strong risk factor for the subsequent development of an SUD and can jeopardize drug treatment. Assessment for ADHD is highly recommended amongst SUD patients as is a drug evaluation for those adults diagnosed with ADHD. An undiagnosed comorbidity can result in poor results as only part of the problem is treated. More research is needed to clarify relationship between adult ADHD and substance abuse, as well as to explore new psychopharmacological and psychotherapeutic treatments for this comorbidity.