The Australian Shepherd, often known simply as the “Aussie”, is a medium-sized breed of dog that was, despite its name, developed on ranches in the Western United States during the 19th century. The dog was developed from a breed or breeds from the Basque country in Western Europe. It was brought by Basque people to the United States. These people had previously lived only briefly in Australia before moving to America. The breed otherwise has no connection to Australia.

There are a number of different theories regarding how the breed came to be associated with Australia, but there is no consensus. They are similar in appearance to the popular English Shepherd and Border Collie breeds, and research has found that Australian Shepherds and Border Collies are closely related to each other; both the Border Collie and Australian Shepherd are slightly more distantly related to other kinds of Collies and to Shetland Sheepdogs.

Australian Shepherds rose in popularity with a boom in Western riding after World War II.They became known to the general public through rodeos, horse shows, and Disney movies made for television.

For many years, Aussies have been valued by stockmen for their versatility and trainability. While they continue to work as stock dogs and compete in herding trials, they have earned recognition in other roles due to their trainability and eagerness to please and are highly regarded for their skills in obedience. Like all working breeds, Aussies have considerable energy and drive and usually need a job to keep them occupied. They often excel at dog sports such as dog agility and frisbee and are also highly successful as search and rescue dogs, disaster dogs, detection dogs, guide dogs, service dogs, and therapy dogs. They are considered the 17th-most popular dog breed in the United States.

The Australian Shepherd is traditionally a medium-sized breed of solid build. They can weigh from 30 to 65 pounds (14 to 29 kg) and stand from 17 to 26 inches (43 to 66 cm) in height. The Australian Shepherd Club of America (ASCA) standard calls for the Australian shepherd to stand between 18 and 23 inches (46 and 58 cm) at the withers, females being 18 to 21 inches (46 to 53 cm) and males measuring 20 to 23 inches (51 to 58 cm); however, quality is not to be sacrificed in favor of size.[2]

Recently, Australian Shepherds have been selectively bred in favor of size to develop Miniature and Toy Aussies. The American Stock Dog Registry (ASDR) now recognizes three sizes. The above description is considered a Standard Aussie (height 18 to 23 inches for males and females). Miniature Australian Shepherds (or Mini Aussies) stand 14 to 18 inches tall, while Toy Aussies stand between 10 and 14 inches.