Abstract. This paper deepens the connections between critically finite rational maps and finite subdivision rules. The main theorem is that if f is a critically finite rational map with no periodic critical points, then for any sufficiently large integer n the iterate f •n is the subdivision map of a finite subdivision rule. This enables one to give essentially combinatorial models for the dynamics of such iterates.We are interested here in connections between finite subdivision rules and rational maps. Finite subdivision rules arose out of our attempt to resolve Cannon's Conjecture: If G is a Gromov-hyperbolic group whose space at infinity is a 2-sphere, then G has a cocompact, properly discontinuous, isometric action on hyperbolic 3-space. Cannon's Conjecture can be reduced (see, for example, the Cannon-Swenson paper [5]) to a conjecture about (combinatorial) conformality for the action of such a group G on its space at infinity, and finite subdivision rules were developed to give models for the action of such a Gromov-hyperbolic group on the 2-sphere at infinity.There is also a connection between finite subdivision rules and rational maps. If R is an orientation-preserving finite subdivision rule such that the subdivision complex S R is a 2-sphere, then the subdivision map σ R is a critically finite branched map of this 2-sphere. In joint work [3] with Kenyon we consider these subdivision maps under the additional hypotheses that R has bounded valence (this is equivalent to the subdivision map not having periodic critical points) and mesh approaching 0. In [3, Theorem 3.1] we show that if R is conformal (in the combinatorial sense), then the subdivision map σ R is equivalent to a rational map. The converse follows from [4, Theorem 4.7]. While a finite subdivision rule is defined in terms of a map from the subdivision complex to itself, it is basically a combinatorial procedure for recursively subdividing polygonal 2-complexes. By establishing the equivalence between a rational map and a subdivision map, one is giving an essentially combinatorial description of the action of the rational map.In this paper we consider the problem of when a rational map f is equivalent to the subdivision map of a finite subdivision rule. Since a subdivision complex has only finitely many vertices, such a rational map must be critically finite. We specialize here to the case that f has no periodic critical points. Our main theorem,