Effiziente Lokalisierung von Nutzern und Geräten in Smarten Umgebungen The thesis considers determination of location of sensors and users in smart environments using measurements of Received Signal Strength (RSS). The first part of the thesis focuses on localization in Wireless Sensor Networks and contributes two fully distributed algorithms which address the Sensor Selection Problem and provide the best trade off between energy consumption and localization accuracy among the algorithms considered. Furthermore the thesis contributes to Device Free Localization an indoor localization concept providing scalable and highly accurate location estimates (prototype: 0.36mø MSE) while using a COTS passive RFID System and not relying on user carried sensors.