The reason for the stacking fault might be Friedel oscillations causing an increased charge density pocket at the fcc site for smaller magnesium clusters. Terrace self-diffusion of single atoms runs almost barrier-free on Mg(0001) but with higher activation energies on Mg(101 ! 1). When adsorbates collide on the surfaces, initial dimer, trimer, and cluster structures are formed, which is highly favored due to high attractive interactions between the adsorbates. The small cluster structures can be seen as seeds for further island growth, which shape on Mg(0001) could be either triangular with only <1 ! 1 ! 20>steps as proposed by Jäckle et al. or hexagonal with alternating <112 ! 0>-and <1 ! 1 ! 20>-steps at elevated temperatures . However, the island pattern should be compact rather than fractal due to low step-edge and corner crossing barriers. On Mg(101 ! 1), the diffusion barriers are in general higher compared to Mg(0001), but the ratios for bond formation and breakage are closer together. All processes studied on Mg(101 ! 1) can occur at ambient conditions, while on Mg(0001) kink evaporation and the formation of a step-vacancy are not activated at room temperature. Surface growth on Mg(101 ! 1) could proceed in a line along the hollow sites perpendicular to the [101 ! 2]-direction. The three-dimensional diffusion studies reveal negative Ehrlich-Schwoebel barriers for all investigated exchange descend processes on Mg(0001) and Mg(101 ! 1) (except [12 ! 10]A-step), indicating a uniform and smooth surface growth due to a high rate of interlayer mass transport at low deposition rates. However, a second adatom significantly increases the descent barriers for hopping and exchange processes, fostering the aggregation of new islands on top of existing islands if the adatom meets another adatom faster than it reaches the step-edge.