MBE is a versatile technique for growing thin epitaxial structures made of semiconductors, metals or insulators [7.1] (see also the definition given in Sect. 6.1.3). What distinguishes MBE from previous vacuum deposition techniques is its significantly more precise control of the beam fluxes and growth conditions. Because of vacuum deposition, MBE is carried out under conditions far from thermodynamic equilibrium and is governed mainly by the kinetics of the surface processes occurring when the impinging beams react with the outermost atomic layers of the substrate crystal. This is in contrast to other epitaxial growth techniques, such as LPE or atmospheric pressure VPE, which proceed at conditions near thermodynamic equilibrium and are most frequently controlled by diffusion processes occurring in the crystallizing phase surrounding the substrate crystal.