A police dog, also known as a K-9, is a dog that is trained to assist police and other law enforcement officers. Their duties may include searching for drugs and explosives, locating missing people, finding crime scene evidence, protecting officers and other people, and attacking suspects who flee from officers. The breeds most commonly used by law enforcement are the German Shepherd, Belgian Malinois, Bloodhound, Dutch Shepherd, and Labrador Retriever. In recent years, the Belgian Malinois has become the leading choice for police and military work due to their intense drive, focus, agility, and smaller size, though German Shepherds remain the breed most associated with law enforcement.

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.