An assistance dog, known as a service dog in the United States, is a dog trained to aid or assist an individual with a disability. Many assistance dogs receive training from a handler or from an assistance dog organization.

The dog is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from extinct gray wolves, and the gray wolf is the dog's closest living relative. The dog was the first species to be domesticated by humans. Experts estimate that hunter-gatherers domesticated dogs more than 15,000 years ago, which was before the development of agriculture. Due to their long association with humans, dogs have expanded to a large number of domestic individuals and gained the ability to thrive on a starch-rich diet that would be inadequate for other canids.