Mixed breed dogs can vary in the complexity of their heritage – from the most simple first generation cross breed mix of two pedigree breeds (for example, the Cockapoo, which is the offspring of a Cocker Spaniel and a Poodle) to outbred dogs that live in freely interbreeding populations in the wild.

Outbred or wild dogs have no recent ancestry from pedigree dog populations, and can be considered to be true mixed breed dogs. In many outbred dogs, all 8 grandparents were also mixed breed dogs.

Mixed breed dogs vary greatly in size, shape, and color, and consequently, are often difficult to classify physically.