Mental retardation is a specialist area of clinical psychology practice that aims to promote improved quality of life for individuals with mental retardation and their families. To promote improved quality of life for children and adolescents with mental retardation and their families, clinical psychologists require a range of basic and expert competencies. This chapter delineates essential areas of basic and expert competencies that will enable psychologists to be more successful in this specialized area of clinical practice. Emphasis is given to the implementation of evidence-based assessments and treatments for children and adolescents with mental retardation. With respect to assessment, the basic competencies include: (a) recognition and assessment of symptoms, (b) familiarity with contemporary definitions and classification systems, (c) intelligence testing, (d) adaptive behavior assessment, and (e) an understanding of how genetic syndromes, learning impairment, operant conditioning, environmental impoverishment, and social barriers influence behavior and development. With respect to treatment, competencies are required with respect to a range of contemporary evidence-based treatment approaches, including: (a) behavioral intervention, (b) cognitive-behavior therapy, (c) family therapy, (d) early intervention, (e) special education, (f) supported employment, and (g) pharmacological treatments. Effective use of these treatment approaches requires skillful implementation and an understanding of the basic mechanisms of change that underlie the approach. Enhancing the quality of life for some children and adolescents with mental retardation and their families will often require additional expert competencies in specialist areas, such as providing culturally-sensitive practices, treating severe problem behavior, and promoting learning in children with profound/multiple impairments. The transition from basic competence to expert clinician requires a planned and systematic approach for gaining relevant new knowledge and clinical skills. By gaining basic and expert competencies, clinicians can greatly enhance the quality of life of children and adolescents with mental retardation and their families.