People with intellectual and developmental disability (IDD) experience the full range of mental health conditions as the general population. Yet, they are often underdiagnosed and misdiagnosed due to lack of understanding of the factors that influence effective assessment and treatment. The underlying deficits in IDD can create misunderstanding of signs and symptoms. This presentation will focus on some of the most common challenges associated with assessing and diagnosing mental health conditions for individuals with ID including the role of biopsychosocial factors.