Parent-of-origin effects were first recorded >3000 years ago by mule breeders in Asia Minor. There are now several different types of evidence suggesting the presence of a large number of imprinted genes, many of which have not yet been identified. Here, we catalogue a wide range of evidence and phenomena which indicate or suggest the presence of genomic imprinting in animals. This evidence includes: the direct documentation of parent-of-origin-specific gene transcription; human disease inheritance patterns which suggest the involvement of imprinted genes; and older, less well studied animal models which may show parent-of-origin effects.