Inbreeding is the mating of closely-related animals to produce the next generation of progeny. Some people try to make a distinction between inbreeding and what is called line-breeding but although the distinction is popular with dog-breeders it is not recognised by geneticists. Inbreeding is traditional and popular with dog-breeders, particular in dog show circles, because it represents a quick and easy way to fix exaggerated points of "type" and appearance, colours, markings, etc. It is also used as a tool to quickly bring about "genetic stability" in new breeds.

Inbreeding soon results in increased levels of homozygosity and a consequent reduction in genetic diversity within the inbred population. That in turn reduces the adaptability of the animals in the face of new environmental challenges or of natural and/or artificial selection. Purebred dog breeds that have been inbred for long periods of time show a considerable lack of genetic diversity along with high levels of genetic diseases.

Relative levels of inbreeding may be gauged with reasonable accuracy when the full pedigrees of the animals in question are known. This is done using Wright's Coefficient of Inbreeding (usually called COI).