RFID-tags can be seen as a new generation of bar codes with added functionality. They are becoming very popular tools for identification of products in various applications such as supply-chain management. The widespread deployment of RFID technology will depend to a large extent on its acceptance by the general public. Thus, developing privacy and security technologies specifically suited to the constrained environment of RFID tags continues to be a key problem. In this paper, we introduce several new mechanisms that are cheap to implement or integrate into RFID tags and that at the same time enhance the security of the tags and the privacy of the individual carrying the tags. These new mechanisms are based on physical principles alone or on their combination with cryptographic methods. We also review previous works that use physical principles to provide security and privacy in RFID systems. Key Words. RFID, privacy, cheap security solutions, sensors, physics and cryptography