Contemporary policy in the care of persons with developmental disabilities is focused on "social inclusion" and equity. Healthcare professionals in the mainstream are tasked to ensure that their services are both available and responsive to caring for individuals. This clinical guide, written by a psychiatrist and a clinical psychologist with clinical and academic expertise, aims to outline relevant knowledge, skills, and attitudes needed to promote better health outcomes for people with developmental disabilities. The guide is organized into three sections and includes learning objectives and self-examination questions on content. The first section on knowledge covers definitions, etiological factors, health problems, mental health problems, and the service needs of individuals and their caregivers as they evolve over the lifespan. Section two focuses on skills including assessment, case formulations, interprofessional collaboration, and the provision of developmental services, psychosocial treatments, and biomedical treatments. The final section reviews attitudes related to the provision of empathetic support, respect for interprofessional collaboration, vigilance about neglect and abuse, and various special challenges in providing care. The authors also address informed consent, sexuality, parenting, and individuals in conflict with the law.