The term 'K9' or 'K-9' is derived from the English and French word 'CANINE' (which simply means 'DOG'). 'CA'='K' and 'NINE'='9' (same as 'YOU TOO'='U2'). The first use of 'K-9' goes probably back to 1942. In that year the first military K-9 Corps were created by US Secretary of War Robert P. Patterson.

Dogs have an amazing sense of smell and can smell for hidden suspects better and faster than humans can look for them with their eyes. Using K9s is also safer for human officers since a K9 can alert to the presence of a potentially armed suspect from a far distance away.

The most popular breeds are German Shepherds, Belgian Malinois, Dutch Shepherds, and occasionally mixes of these breeds. Less popular, but still used at times, are Rottweilers, Doberman Pinchers, and Bouvier de Flandres. Other breeds are used for detection work, if this is their sole purpose.

When a dog is protecting a person, he's reacting to a dog or person approaching the pet owner and himself. Protective dog behavior manifests differently for different dogs. Dogs will either freeze, glare at the approaching person, snarl, show teeth, snap or even bite.