The chapter focuses on multicultural issues in the definition, epidemiology, diagnosis, assessment, and treatment of racially and ethnically diverse clients with intellectual disabilities (IDs). Prevalence rates of mental health problems for persons with IDs are significantly higher than their peers without IDs. Psychological assessment and treatment are especially complex with persons from culturally diverse groups with IDs. Despite the growing recognition of the importance of considering diversity issues, there is a paucity of appropriate assessment measures and methods, evidence-based interventions, and clinicians trained to address the mental health needs of racially and ethnically diverse people with IDs and related disabilities. Recommendations for future directions and research are discussed.