The questions of surface structure can be roughly divided into the ``large scale'' and the ``small scale'' structure. The two are divided by an arbitrary and unclear borderline. Some interesting and more macroscopic phenomena still lack a detailed understanding on the microscopic level, for example catalysis or friction. Here we discuss the ``large scale'' structure only very briefly in connection with surface free energies and crystal shape. We also mention some very interesting phenomena such as the roughening transition and surface melting . This discussion is done in the framework of thermodynamics. All the rest of the chapter is concerned with the microscopic structure. We are interested in questions like: What is the structure of clean and adsorbate-covered surfaces? Why is the structure like this? How does one measure the structure?