I use computer simulations to examine the evolution of close galaxy pair counts and the formation of galaxy clusters. I show that the evolution of the close pair fraction is much weaker than the strong evolution in the dark matter halo merger rate. I continue with a discussion of how galaxy cluster-sized dark matter halos are assembled. This result, contrary to some expectations, shows that galaxy clusters in simulations are built from field halos instead of groups. I conclude by presenting preliminary results of a method to determine the pair fraction of galaxies at z=3 and apply it to a spectroscopic sample of LBGs. I show that the pair fraction is a factor of ~ 4 higher at z ~ 3 compared to local galaxy samples with similar number densities.