There are numerous dog breeds, which evolved during the domesticated dog's relationship with man over the last 100,000 years. Many breeds are the product of a deliberate process of artificial selection. Because of this, some breeds are highly specialized, and there is extraordinary morphological diversity across different breeds. Despite these differences, dogs are able to distinguish dogs from other kinds of animal.

The definition of a breed is a matter of some controversy. Some groups use a definition that ultimately requires extreme in-breeding to qualify. Dogs that are bred in this manner often end up with severe health problems. Other organizations define a breed more loosely, such that an individual may be considered of one breed as long as, say, three of its grandparents were of that breed. These considerations come into play among breeders who enter their dogs in dog show competitions.

Dog breeds can be divided into different families, such as:

Herding
Hound
Sighthound
Scenthound
Mastiff
Pointer
Retriever
Spaniel
Spitz
Terrier
Toy