According to the hierarchical scenario, galaxies form via merging and accretion of small objects. Using N-body simulations, we study the frequency of merging events in the history of the halos. We find that at z<~2 the merging rate of the overall halo population can be described by a simple power law, (1+z)3. The main emphasis of this paper is on the effects of environment of halos at the present epoch (z=0). We find that the halos located inside clusters have formed earlier (∆z~1) than isolated halos of the same mass. At low redshifts (z<1), the merger rate of cluster halos is 3 times lower than that of isolated halos and twice as low as that of halos that end up in groups by z=0. At higher redshifts (z~1-4), progenitors of cluster and group halos have 3-5 times higher merger rates than isolated halos. We briefly discuss the implications of our results for galaxy evolution in different environments.