Radio Frequency Identification (RFID) systems have become omnipresent and played important roles in both business and private sectors. The increasing reliance on RFID systems for a wide range of applications arises the need to thoroughly investigate the related security mechanisms. Although most modern RFID devices employ cryptographically strong algorithms (e.g., (Triple-)DES or AES) to provide various security services (e.g., encryption, authentication, etc.), the actual hardware realization of RFID devices is subject to side-channel attacks (SCA) that utilize physical properties to break analytically secure cryptographic primitives. In this chapter, we describe a general framework for side-channel attacks on RFID devices and discuss recent progress of applying side-channel attacks to break real-world RFID systems.