Most dogs today are kept as pets rather than for any particular work. Both guard dogs and watch dogs bark to alert their owners of an intruder's presence and scare away the intruder. The watch dog's function ends here; a guard dog when properly trained, is capable of attacking or restraining the intruder. For example, livestock guardian dogs are often large enough and strong enough to attack and drive away predators such as wolves. In cases where intruders are more likely to be human, guard dogs are sometimes trained simply to restrain an intruder with his owner's guidance, as in Schutzhund or K9 Pro Sports training.

Many of the now prominent guardian breeds such as Rottweilers started as farm dogs types but then developed over many years into guard breeds. Some breeds, such as the Weimaraner and Rhodesian Ridgeback, were originally bred for hunting, but their large intimidating look and territorial instincts have helped them evolve into guard dogs in today's society. Others like Dobermans were specifically and originally bred as guard dogs