People with intellectual disability (ID) are more susceptible to mental disorders including mental illness and developmental disorders, such as autism and attention-deficit hyperactivity disorder (ADHD). The identification of comorbidities is an important task for the clinician as, in addition to clarifying the diagnosis, it carries implications for the treatment and prognosis. This chapter aims to explore the association between ID and ADHD and review the key research findings. The validity of the diagnosis of ADHD in ID is considered. The assessment issues relating to the diagnosis of ADHD in this population are discussed. The questions around treatment and management are summarized and, finally, the clinical implications are highlighted.