According to the World Canine Organization FCI (Fédération Cynologique Internationale), there are approximately 400 recognized breeds of dog worldwide. There is some contention, however, as to the exact number of dog breeds as various kennel clubs recognize different breeds. In addition, all "purebreds" are, in reality, derived from mixed-breed dog populations, complicating the issue further.

One of the oldest breeds of dogs, Chow Chows are known for their bluish-black tongues and gums. They aren't born with this coloration, however. At birth, a Chow Chow's tongue and gums will be pink; by eight weeks, however, they will take on the bluish-black color. Originally from China, the Chow Chow's name (Songshi Quan), literally translated, means "puffy lion dog".

Standing at an average height of approximately 34 inches (from the withers or shoulder-blade), the Irish Wolfhound is generally considered the world's largest (tallest) dog. According to legend, Irish Wolfhounds were guardians and companions of ancient Irish kings and were valued by Celtic chieftains as dogs of war. According to some sources, they were (as the name implies) originally bred to hunt wolves. In spite of their impressive height, the average weight for an Irish Wolfhound is only about 120 pounds. The English Mastiff is generally considered the world's heaviest dog. In 1999, an English Mastiff named Kell weighed in at a whopping 286 pounds (130 kg). According to Kell's owner, her food bill came to £200 a week!

The German Shepherd Dog was originated in Germany in the late 1800s by Captain Max von Stephanitz, a former German Army Captain who hoped to breed an all-purpose working dog. He created the schutzhund trial, a breed test, and any dog that failed the trial was prohibited from breeding. These strict guidelines led to the rapid development of the breed's abilities and intelligence, and the German Shepherd soon proved itself a superior working dog. After World War I, American soldiers, impressed by the German Shepherd's intelligence and trainability, brought the breed to the United States where it quickly became a popular pet as well as working dog. Today, German Shepherds are often trained as police dogs, guide dogs, guard dogs, and of course sheepdogs. They are also known as Alsations or Schaferhunds.

Reaching speeds of up to 45 miles per hour (72 km/h), the Greyhound is the fastest breed of dog. In fact, the only land animal faster than the Greyhound is the cheetah. Not surprisingly, Greyhounds are bred primarily for racing.

The Basenji, an African hunting dog, is the only dog that cannot bark. This trait is probably due to the shallow laryngeal ventricle in its larynx. However, the Basenji is certainly not silent and can make a wide variety of other dog noises as well as a unique yodelling sound. One of the most ancient breeds of dogs, Basenjis were once feared extinct until they were rediscovered living among the Azande and Mangbetu tribes from the northeastern Congo region of Africa in 1895.

According to the American Kennel Club's registration statistics, the most popular dog in the United States is the Labrador Retriever. In 2004, nearly three times as many Labs were registered (146,692) as any other breed. This marked the 15th consecutive year that the Labrador Retriever topped the rankings. Golden Retrievers (52,550 registrations) were the second most popular breed of dog, followed by German Shepherds (46,046), Beagles (44,555), and Yorkshire Terriers (43,522). According to the AKC's statistics, the least popular purebred dog in 2004 was the English Foxhound with only 17 registrations.

The Dobermann or Dobermann Pinscher was originated in Germany around 1890 by Louis Dobermann, a tax collector who needed a guard dog for protection. Hoping to develop a breed of dog with maximum strength, loyalty, intelligence, and fierceness, Dobermann is believed to have mixed a number of breeds including the German Shepherd, the Great Dane, the Greyhound, the Pinscher, and the Rottweiler. Today, Dobermanns are commonly used as guard dogs and police dogs.

Basset Hounds are generally considered to be the breed of dog with the longest ears. According to the Guinness Book of World Records, a Basset Hound from Germany named Jack holds the record for longest ears on a dog. Jack's ears measure a remarkable 13 inches and are so long that he sometimes trips over them when he walks.

All modern dogs are generally believed to be descended from Tomarctus, a stealthy predator that roamed the plains and grasslands of North America approximately ten to fifteen million years ago. Wolves, jackals, and coyotes are also believed to be descendants of Tomarctus.