The term severe disability is primarily used in reference to people with severe to profound cognitive and adaptive functioning impairments associated with intellectual disability, autism spectrum disorder, and/or cerebral palsy. However, the term severe disability could also be applied to some individuals with traumatic brain injury and various neurodegenerative conditions, such as Alzheimer’s, amyotrophic lateral sclerosis, and other motor neuron diseases. This chapter describes a range of conditions associated with severe disability. The cognitive and adaptive functioning limitations of people with severe disabilities, and associated learning and behavioral characteristics, are also reviewed.