I want to discuss some problems about explanation in biology. The leading question about biological explanation has been the question whether, and if so how, it differs from explanation in other fields. Many claims and counterclaims are made by biologists as well as philosophers in this connection. I want to examine in particular the claim of Francisco Ayala that biology is irreducible to physics and chemistry because, and only because, of the teleological structure of evolutionary theory.¹ But to reflect on Ayala's claim I must have some notion of what makes a theory in general explanatory and also some notion of what makes an explanation teleological. While I can't claim to have a pat answer on either of these puzzles (especially the former!), let me make some preparatory remarks about both before looking at the alleged teleology of evolutionary theory in relation to its explanatory force.