THE HOMOTOPY THEORY OF FUSION SYSTEMS by Carles Broto, Ran Levi, and Bob Oliver We define and characterize a class of $p$-complete spaces $X$ which have many of the same properties as the $p$-completions of classifying spaces of finite groups. For example, each such $X$ has a Sylow subgroup $BS--->X$, maps $BQ--->X$ for a $p$-group $Q$ are described via homomorphisms $Q--->S$, and $H^*(X;F_p)$ is isomorphic to a certain ring of ``stable elements'' in $H^*(BS;F_p)$. These spaces arise as the ``classifying spaces'' of certain algebraic objects which we call ``$p$-local finite groups''. Such an object consists of a system of fusion data in $S$, as formalized by Ll. Puig, extended by some extra information carried in a category which allows rigidification of the fusion data. Since we distributed preprints of this paper, we have made several major changes to its terminology; as well as some changes to the statement and proof of Proposition 9.1. Any suggestions or comments people might want to make on the new terminology will be welcome.