An overview of various cleaning procedures for silicon surfaces is presented. Because in-situ cleaning becomes more and more important for nanotechnology the paper concentrates on physical and dry chemical techniques. As standard ex-situ wet chemical cleaning has a significant impact on surface quality und thus device properties, its influence on further processes is also considered. Oxygen and carbon are unavoidable contaminations after wet chemical treatment and therefore we discuss their in-situ removal as one of the main goals of modern silicon substrate cleaning. As surface roughness strongly influences the electrical quality of interfaces for epitaxy and dielectric growth, we concentrate on techniques, which meet this requirement. It will be shown that multi-step thermal sequences in combination with simultaneous passivation of the clean surface are necessary in order to avoid recontamination. This can be achieved not only for ultra hich vacuum but also for inert gas atmosphere. In this case the process gases have to be extremely purified and the residual partial pressure of contaminats such as oxygen and carbon has to be negligible. It will be demonstrated that 800°C is an upper limit for thermal treatment of silicon surfaces in the presence of carbon because at this temperature SiC formation in combination with a high mobility of silicon monomers leads to surface roughness. In addition mechanical stress causes dislocations and crystal defects.