This chapter provides an introduction to structural defects at surfaces. A surface defect may be defined as an arrangement with lower symmetry than that of the relaxed, reconstructed or rearranged zero-T equilibrium surface. In this sense, also thermal disorder and disorder related with surface phase transitions represent defective structures. Clean surfaces are prepared either by cleaving which often leaves the surface in a state far from equilibrium or by ion bombardment and annealing or in-situ growth. Annealed or grown surfaces represent at best the equilibrium structures at an often unknown freezing temperature at which the atomic mobility during cooling becomes too low. There exist even systems where atomic steps which one spontaneously would consider as defects are part of the zero-T equilibrium structure. Therefore, in common use the term “defect” corresponds rather to a structure which represents a deviation from that what is believed to be an ideally relaxed or reconstructed long-range ordered surface structure. More systematically, defects may be described according to their dimension. New classes of defects exist for non-primitive crystals (with more than one atom in the unit cell), if a surface plane does not contain all atoms of a unit cell. For compounds, antisite defects are possible (sites occupied by atoms different from bulk prediction). For alloys, the surface composition or surface arrangement may differ from the bulk. Stereographic triangles for cubic and hexagonal crystals of the hcp type as well as the wurtzite-A-faces with the three low Miller-index poles (LIPS) are given.