American Staffordshire terriers are on the larger side for medium-size dogs, with female Am Staffs typically being slightly smaller than the males.
The Australian cattle dog is an active and intelligent dog breed that is right in the middle of the medium-size category.
The Australian shepherd is a very intelligent, high-energy, medium-size dog breed.
The basset hound is an easygoing dog with a distinct appearance. While these dogs are built low to the ground with short legs, they still weigh solidly in the medium-size range.
The happy-go-lucky beagle is a favorite among those who want a dog that is moderately active and very friendly and sweet.
The bearded collie is a clever and outgoing dog breed. Hovering at around 50 pounds, the dog is actually smaller than it appears to be.
The border collie is one of the smartest dog breeds as well as a high-energy pup.
The Boykin spaniel is a friendly, alert dog that makes an excellent bird dog and a relatively easygoing house pet.
The Brittany is another medium-size dog that excels at hunting. This high-energy breed thrives on physical activity, whether is it hunting or a dog sport such as agility.
The bulldog is a stout dog known for its short muzzle and tendency to snore.