Free navigation in indoor environments is one of the main enabling technologies for many service robot applications. Siemens has developed SINAS, a navigation system which currently is primarily targeted towards cleaning robot applications. Its suitability for tough everyday operation has been successfully demonstrated since August 1996 on several occasions, e.g. in several chain store supermarkets. The paper discusses the main requirements of a navigation system for cleaning robots, presents the structure and main features of the SINAS system, and reports experiences and results from the field tests.