Every real crystal must have at least one imperfection -its surface. It is useful to introduce crystal interfaces by thinking of a surface that is parallel to a prominent crystallographic plane and represents a smooth sheet of atoms, of which the pattern is the same as that of a parallel plane inside the crystal. We shall use this simple clear idea, but recognize it is not strictly true. Surfaces show the phenomenon of reconstruction, whereby even in high vacuum the outer layers of atoms rearrange into a more energetically favourable situation. A well-known example is the 7 × 7 structure found on (111) Si [1]. Similar surface reconstructions occur on clean metal surfaces [2]. These reconstructions can be rationalized in terms of the coordination of the atoms at the surface and the electronic structure at the surface being different from the bulk (which, in practice, typically means distances of > 1 nm from the surface).Rather than such outer surfaces, we are mainly concerned in this chapter with internal interfaces in crystalline solids, for example grain boundaries and boundaries between different phases, such as epitaxial interfaces, although we begin in this section with the concept of surface free energy to achieve this. The topic of internal interfaces covers a very wide field [3], from which we have attempted here to distil the essential aspects relevant to a basic understanding of the crystallography of internal interfaces. Books listed in the Suggestions for Further Reading direct the reader to more advanced treatments of this topic.We shall formally define a flat surface parallel to any given rational plane by removing all the atoms whose centres lie to one side of a plane of this orientation located within the bulk crystal. With this assumption, in highly symmetrical crystal structures with one atom per lattice point, such as the c.c.p. and b.c.c. crystal structures, the location of the defining plane will not affect the structure of this idealized surface, because each atom has identical surroundings within the bulk.