This paper unfolds the recent concept of using harmonic signals generated by the nonlinear behavior of traditional passive UHF RF identification (RFID) chips. The harmonics generation of passive RFID chips is analyzed to highlight the influences of the matching network and the tag antenna. The proposed study relies on a theoretical model that explains the role in the harmonic production of different elements constituting the tag. The model also shows how to achieve a second communication channel exploiting the third harmonic frequency in addition to the fundamental frequency channel. Specifications and methodology are explained in a harmonic communication scenario. The regulations, harmonic RFID reader considerations, harmonic tag guidelines design, and metrics for its evaluation are discussed. In compliance with the standard regulations EPC Class-1 Gen2 RFID, a design example is presented. The performance of the solution is illustrated from simulation and measurements. The prototype example operates simultaneously at fundamental and third harmonic frequencies with a read range greater than 4.5 m.