The description of crystal surfaces requires some basic knowledge of crystallography. Therefore, this chapter presents a short overview of crystal lattices and their classification due to symmetry. This knowledge is required to understand the substrate structure and the orientation of the surface. However, the 3-D point groups, space groups and the mathematical description of symmetry operations in three dimensions are not described here: for a more detailed explanation the reader is referred to the International Tables of Crystallography [2.1], which is the standard reference book, or a number of textbooks on crystallography published by the International Union of Crystallography [2.2–2.5]. The 2-D space groups and symmetry operations are explained with somewhat more detail here because these are frequently used in surface structure determination. A very detailed description of the geometry of crystal surfaces is given in a recent book by K. Hermann [2.6]. A short introduction into the kinematic theory of diffraction and into diffraction at 2-D periodic lattices is also included here.