  In this paper I will review twenty years of work on the question: When is
there coexistence in stochastic spatial models? The answer, announced in
Durrett and Levin [Theor. Pop. Biol. 46 (1994) 363--394], and that we explain
in this paper is that this can be determined by examining the mean-field ODE.
There are a number of rigorous results in support of this picture, but we will
state nine challenging and important open problems, most of which date from the
1990's.
