We study the formation of galaxy cluster-size dark matter halos (M > 1014.0 Msun) formed within a set of cosmological Lambda CDM N-body simulations, and track the accretion histories of cluster subhalos. We use the merger trees of these halos to trace their assembly and examine the populations of clusters/protoclusters in several different redshift intervals. Galaxy clusters provide an interesting environment for the study of several astrophysical phenomena, such as ram pressure stripping. They are the largest collapsing objects in the universe, and, as such, they are an observable constraint of hierarchical structure formation. Through understanding the formation of clusters, we probe key parameters in cosmology and such studies allow us to place constraints on several processes in the formation and evolution of galaxies as well as large scale structure.