There are various situations in which it is natural to ask whether a given collection of k functions, ρ j (r 1 , . . . , r j ), j = 1, . . . , k, defined on a set X, are the first k correlation functions of a point process on X. Here we describe some necessary and sufficient conditions on the ρ j 's for this to be true. Our primary examples are X = R d , X = Z d , and X an arbitrary finite set. In particular, we extend a result by Ambartzumian and Sukiasian showing realizability at sufficiently small densities ρ 1 (r). Typically if any realizing process exists there will be many (even an uncountable number); in this case we prove, when X is a finite set, the existence of a realizing Gibbs measure with k body potentials which maximizes the entropy among all realizing measures. We also investigate in detail a simple example in which a uniform density ρ and translation invariant ρ 2 are specified on Z; there is a gap between our best upper bound on possible values of ρ and the largest ρ for which realizability can be established.