Definitions vary on what a working dog is, they are sometimes described as any dog trained for and employed in meaningful work; other times as any dog whose breed heritage or physical characteristics lend itself to working irrespective of an individual animal's training or employment; and other times again it is used as a synonym for herding dog.   
Roles performed by dogs that sometimes sees them classified as working dogs include:
- Assistance dog 
- Detection dog 
- Draught dog 
- Guard dog 
- Guide dog 
- Herding dog 
- Military working dog 
- Search and rescue dog 
- Service dog 
- Sled dog 
- Therapy dog 
When competing in conformation shows, a number of kennel clubs classify various pedigree dog breeds into a "working group" or "working dogs group", although it varies between kennel club what breeds are so classified. The Kennel Club classifies mastiff, pinscher, sled dog, select livestock guardian dog and some other breeds in their working group.  The American Kennel Club's working group and the Canadian Kennel Club's working dogs group are very similar to The Kennel Club's except they include all livestock guardian dog breeds and all full sized spitz breeds (not just sled dogs) recognised by the clubs.   Both the Australian National Kennel Council's and New Zealand Kennel Club's working dogs groups comprise herding dog breeds and select livestock guardian dog breeds recognised by those clubs.   The Fédération cynologique internationale has no working group. 
- Working animal – Domesticated animals for assisting people
"working dog". Collins Dictionary. HarperCollins Publishers L.L.C. 2020. Retrieved 21 January 2020.
a dog of suitable breed or training kept for its practical use, such as herding sheep, rather than as a pet or for showing
"working dog". Merriam-Webster. 2019. Retrieved 21 January 2020.
a dog suitable by size, breeding, or training for useful work (such as draft or herding) especially as distinguished from one suitable primarily for pet, show, or sporting use
"working dog". Lexico. Oxford University Press. 2019. Retrieved 21 January 2020.
A dog trained to work with livestock.
- Olson, Patricia N. (1 August 2002). "The modern working dog—a call for interdisciplinary collaboration" (PDF). Journal of the American Veterinary Medical Association. 221 (3): 352–355. Retrieved 22 January 2020.
"Working dog". Encyclopædia Britannica. Encyclopædia Britannica, Inc. 2019. Retrieved 21 January 2020.
any of various breeds of dog bred as guard, herding, draft, or rescue animals.
- Rogak, Lisa (2011). The dogs of war: the courage, love and loyalty of military working dogs. New York: Thomas Dunne Books. ISBN 9781250008817.
- Coppinger, Raymond; Schneider, Richard (1995). "Evolution of working dogs". In Serpell, James (ed.). The domestic dog: its evolution, behaviour and interactions with people. Cambridge: Cambridge University Press. pp. 21–50. ISBN 0521425379.
- "Working Group". The Kennel Club. The Kennel Club Ltd. 2020. Retrieved 21 January 2020.
- "Working Group". American Kennel Club. American Kennel Club, Inc. 2020. Retrieved 21 January 2020.
- "Working Dogs". Canadian Kennel Club. 2020. Retrieved 22 January 2020.
- "Group 5 (Working Dogs)". Australian National Kennel Council. Australian National Kennel Council Ltd. 2015. Retrieved 22 January 2020.
- "Standards of the breeds: Group 5 - Working Dogs" (PDF). New Zealand Kennel Club. Dogs New Zealand. 2020. Retrieved 22 January 2020.
- "FCI breeds nomenclature". Fédération Cynologique Internationale. 2020. Retrieved 22 January 2020.