In this paper, I develop some internal problems with Alvin Plantanga's proper functionalist epistemology. I focus on: (1) how we know that a belief is the result of proper function and the special difficulties this occasions for religious beliefs; (2) what a properly functioning person should believe in various circumstances, and (3) the problem of design – whether the claim that God designed us can be reconciled with the claim that He was subject to trade-offs, compromises and unintended by-products. These serious internal problems cast doubt upon proper functionalism's fruitfulness as a theory of knowledge.