We present a study of island formation (the transition from 2D to 3D growth) during the Stranski-Krastanow (S-K) growth of Ge on Si. The energetic driving force for S-K island formation should be the ability to relax the islands by dislocation introduction. Here, we show that Ge islands formed on Si (100) are initially dislocation-free, in the presence of a 2D “sea” that is far in excess of the equilibrium 3 monolayers (ML). We call this phase of growth “dislocation-free S-K”. The 2-D sea does not collapse until dislocated islands are produced at an average coverage of = 7 ML. We call the dislocation-free island phase “coherent S-K” growth. The corresponding 2D-3D transition on Si (111) appears to reach equilibrium far faster, and we have not observed dislocation-free island formation: dislocated islands are seen at = 5 ML. As expected, the kinetics allow us to suppress island formation on (100) by reducing the growth temperature. These thick 2D films are analogous to those grown on As-covered surfaces, but have a microstructure dominated by edge dislocations.