A developmental disability is a permanent disability, present before age 18, that causes challenges with day-to-day living. A developmental disability may be mental, physical, or a combination of the two. People with developmental disabilities experience difficulty with things like personal care, language skills, learning, and independent living, and the difficulties may be mild or severe. People with developmental disabilities may also have challenging behaviours and are often at risk of social isolation.

Most people know someone with a developmental disability. People with developmental disabilities might have Autism Spectrum Disorder (ASD), Down syndrome, Cerebral Palsy, or Fetal Alcohol Syndrome Disorder (FASD), for example. Other people may experience an illness or injury in childhood that leads to a developmental disability.

When a person has a dual diagnosis, he or she has a developmental disability as well as a mental health issue. One resource for families or caregivers supporting a person with a dual diagnosis is the Mental Health Commission of Canada, which recently published the National Guidelines for a Comprehensive Service System to Support Family Caregivers of Adults with Mental Health Problems and Illnesses.