An important goal for computer science is to find practical, scalable models of computation that are as efficient as is permitted by the laws of physics. Physics implies fundamental constraints on the efficiency of computations that produce entropy. As a result, it appears that the most efficient possible computers must use reversible primitive operations which produce arbitrarily little entropy. In this paper we show that a 3-D mesh of reversible processor chips, constructible using existing technology, scales better than any physically possible computer based on irreversible primitives.