This book concerns a new paradigm in the field of UHF RFID systems: the positive exploitation of nonlinear signals generated by the chips integrated into the RFID tags. After having recalled the main principles in RFID technology and its current challenges notably with the emergence of Internet of Things or the smart connected environments, the purpose is to focus on the presence of nonlinearities produced by the nonlinear circuits of RFID chips: effects, nuisances and solutions but also and especially use of the phenomena. The presentation covers all aspects from the characterization of the nonlinear behavior of RFID tags and the associated platforms (distinguishing conducted and radiated measurement) to the design of new types of tags where nonlinearities are exploited in order to offer new capabilities or enhanced performance.