Grain boundaries separate regions of the same phase and crystal structure but diﬀerent orientation. A displacement of a grain boundary is entirely equivalent to the growth of one crystallite at the expense of the shrinking neighbor. In this sense the grain boundary constitutes the contact area of the internal surfaces of adjacent grains. The association of grain boundary motion with the displacement of the internal crystallite surfaces distinctly distinguishes grain boundary motion from a diﬀusive ﬂux of atoms across the boundary. Of course, a non-zero atomic ﬂux across the boundary will make one grain shrink (the emitting grain) and the other grain grow (the receiving grain), and with regard to an external reference frame opposite faces of a bicrystalline specimen would move (Fig. 3.1), but the grain boundary would remain stationary. Thus, diﬀusion across a grain boundary does not necessarily correspond to grain boundary motion with a displacement of crystallite surfaces. It is evident that grain boundary motion consists of the generation of lattice sites at the surface of the growing grain and conversely a destruction of lattice sites at the surface of the shrinking grain. Eﬀectively, grain boundary motion comprises the non-zero net exchange of lattice sites across the boundary (Fig. 3.2).