The Border Collie is a working and herding dog breed developed in the Anglo-Scottish border region for herding livestock, especially sheep. It was specifically bred for intelligence and obedience.

Considered highly intelligent, extremely energetic, acrobatic and athletic, they frequently compete with great success in sheepdog trials and dog sports. They are often cited as the most intelligent of all domestic dogs. Border Collies continue to be employed in their traditional work of herding livestock throughout the world.

In general, Border Collies are medium-sized dogs with a moderate amount of coat, which is most often thick and sheds often. They have a double coat that varies from smooth to rough and is occasionally curled. While black and white is the most commonly seen colour pattern of the Border Collie, the breed appears in just about any colour and pattern known to occur in dogs. Some of these include black tricolour (black/tan/white), liver and white, and red tricolour (red/tan/white) have also been seen regularly, with other colours such as blue, lilac, red merle, blue merle, brindle, and Australian red (also known as ee red, blonde, recessive red, or gold) which is seen less frequently. Some Border Collies may also have single-colour coats.

Eye colour varies from brown to blue, and occasionally eyes of differing colour occur; this is usually seen with merles. The ears of the Border Collie are also variable — some have fully erect ears, some fully dropped ears, and others semi-erect ears (similar to those of the rough Collie). Although working Border Collie handlers sometimes have superstitions about the appearance of their dogs (handlers may avoid mostly white dogs due to the unfounded idea that sheep will not respect a white or almost all white dog), in general a dog's appearance is considered by the American Border Collie Association to be irrelevant. It is considered much more useful to identify a working Border Collie by its attitude and ability than by its looks.

Dogs bred for show are more homogeneous in appearance than working Border Collies, since to win in conformation showing they must conform closely to breed club standards that are specific on many points of the structure, coat, and colour. Kennel clubs specify, for example, that the Border Collie must have a "keen and intelligent" expression, and that the preferred eye colour is dark brown. In deference to the dog's working origin, scars and broken teeth received in the line of duty are not to be counted against a Border Collie in the show ring. The males' height from withers comes from 48 to 56 centimetres (19 to 22 in), females from 46 to 53 centimetres (18 to 21 in).

Border Collies require considerably more daily physical exercise and mental stimulation than many other breeds. The Border Collie is an intelligent dog breed; in fact, it is widely considered to be the most intelligent dog breed. Although the primary role of the Border Collie is to herd livestock, this breed is becoming increasingly popular as a companion animal.

In this role, due to their working heritage, Border Collies are very demanding, playful, and energetic. They thrive best in households that can provide them with plenty of play and exercise, either with humans or other dogs. Due to their demanding personalities and need for mental stimulation and exercise, many Border Collies develop problematic behaviours in households that are not able to provide for their needs. They are infamous for chewing holes in walls, furniture such as chairs and table legs, destructive scraping and hole digging, due to boredom. Border Collies may exhibit a strong desire to herd, a trait they may show with small children, cats, and other dogs. The breed's herding trait has been deliberately encouraged, as it was in the dogs from which the Border Collie was developed, by selective breeding for many generations. However, being eminently trainable, they can live amicably with other pets if given proper socialisation training.

Before taking on the breed as a household pet, potential owners should be sure they can provide regular exercise commensurate with the collie's high energy and prodigious stamina. A working collie may run many miles a day, using its experience, personality and intelligence to control challenging livestock, and these dogs will be distressed and frustrated if left in isolation, ignored or inactive. Like many working breeds, Border Collies can be motion-sensitive and may chase moving vehicles and bicycles, but this behaviour can be modified by training. Some of the more difficult behaviours require patience, as they are developmental and may disappear as the dog matures.

The natural life span of the Border Collie is between 10 and 14 years, with an average lifespan of 12 years. The median longevities of breeds of similar size are usually 12 to 13 years.

Leading causes of death are cancer (23.6%), old age (17.9%) and cerebral vascular afflictions (9.4%).

Hip dysplasia, Collie eye anomaly (CEA), and epilepsy are considered the primary genetic diseases of concern in the breed at this time. CEA is a congenital, inherited eye disease involving the retina, choroid, and sclera that sometimes affects Border Collies. In Border Collies, it is generally a mild disease and rarely significantly impairs vision. However, other eye conditions such as PRA slowly disintegrates the retina and can cause Border Collies to lose almost all of their vision at night which can progress into complete daytime blindness. There is now a DNA test available for CEA and, through its use, breeders can ensure that they will not produce affected pups. There are different types of hip testing available including OFA (Orthopedic Foundation for Animals) and PennHip. Radiographs are taken and sent to these organizations to determine a dog's hip and elbow quality.

Two types of hearing loss occur in the breed. The first type is pigment associated and is found in Border Collie puppies, although the puppies can have congenital sensorineural deafness from birth as well. The second type is known as adult onset hearing loss. These dogs have a normal auditory brainstem response test as pups but gradually lose their hearing some time between one and eight years of age. A study is currently underway at The Translational Genomics Research Institute to identify the genetic cause of adult onset hearing loss in the breed.

Neuronal ceroid lipofuscinosis (NCL) is a rare but serious disease that is limited to show Border Collies. NCL results in severe neurological impairment and early death; afflicted dogs rarely survive beyond two years of age. The mutation causing the form of the disease found in Border Collies was identified by Scott Melville in the laboratory of Dr. Alan Wilton of the School of Biotechnology and Biomolecular Sciences, University of New South Wales. There is no treatment or cure, but a DNA test is now available to detect carriers as well as affected dogs.

Trapped Neutrophil Syndrome (TNS) is a hereditary disease in which the bone marrow produces neutrophils (white cells) but is unable to effectively release them into the bloodstream. Affected puppies have an impaired immune system and will eventually die from infections they cannot fight. The mutation responsible for TNS has been found in Border Collies in English working dogs, in show dogs that had originated in Australia and New Zealand, and in unrelated Australian working dogs. This indicates that the gene is widespread and probably as old as the breed itself. TNS was identified by Jeremy Shearman in the laboratory of Dr. Alan Wilton of the School of Biotechnology and Biomolecular Sciences, University of New South Wales. There is no cure, but a DNA test is now available to detect carriers as well as affected dogs.

Other diseases found less commonly include glaucoma, juvenile cataracts, osteochondritis, hypothyroidism, diabetes mellitus and canine cyclic neutropaenia, carpal soft-tissue injury. A syndrome of exercise induced collapse similar to that seen in Labrador Retrievers (otherwise termed Border Collie Collapse) and triggered by episodes of collapse associated with periods of intense exercise has been described in Border Collies in North America, Europe and Australia; and is currently the subject of further investigation.