Comparative anatomy is the study of similarities and differences in organisms. It is closely related to evolutionary biology (see evolution) and with phylogeny (the evolution of organism development).

Two major concepts of comparative anatomy:

1. Homologous structures are structures (body parts/anatomy) which are similar in different species because the species have a common ancestor. They may not perform the same function. The classic example of this is the forelimb structure of cats and whales.

2. Analogous structures are structures which are similar in different organisms because they evolved in a similar environment and do not have a common ancestor. They usually serve the same purpose or similar purposes. An example is the torpedo body shape of porpoises and sharks. They both evolved in a water environment, but have different ancestors.

Although spoken of less than the above in comparative anatomy and physiology, Heterogeneous structures (structures which are dissimilar), are also present even when there is a common ancestor and a similar environment. For instance the comparative anatomy of dolphins and fish.

The rules for development of special characteristics which differ significantly from general homology were listed by Karl Ernst von Baer (the Baer laws).