A grand canonical Monte Carlo method for the simulation of a simple colloid-polymer mixture called the AO model will be described. The phase separation known to occur in this model is driven by entropy. The phase diagram of the unmixing transition, the surface tension and thecritical point will be determined. To appear in: "Computer Simulation Studies in Condensed Matter Physics XVIII, Eds. D.P. Landau, S.P. Lewis, and H.B. Schuettler (Springer Verlag, Heidelberg, Berlin, 2004).