Evolution is considered by virtually all biologists to be the central unifying principle of biology, yet its fundamental concepts are not widely understood or widely disseminated. Teaching evolution--defined as descent with modification from a common ancestor as a result of natural selection acting on genetic variation--has traditionally been a challenge for most high school biology teachers and has become even more controversial and difficult recently. This is largely due to pressure against teaching evolution by many school boards, school administrators, and parents, and by a highly organized well-funded campaign by creationist groups. The challenge has been intensified by the periodic and recent discoveries of hominid fossils directly linking human origins to the evolutionary process. The purpose of this article is to provide a short summary of some of the most compelling molecular evidence for evolution in hopes that it might be useful to biology teachers at all levels. (Contains 3 figures.)