This chapter presents an overview of many interrelated factors that relate to the successful operation of passive radio frequency identification device (RFID) system. It focuses on far-field passive RFID tags. Two discoveries form the basis for far-field passive RFID tags. First, the development of crystal set radios provides the basis for a tag to power itself. Second, discoveries in the field of radar during the Second World War form the basis for the backscatter communication employed by passive RFID tags. Matching and deliberate mismatching are used for backscatter communication. The reason why the two antenna polarizations are important in RFID is because of the various possible orientations of the item that is tagged relative to the interrogator antenna. The orientation issue in RFID is normally addressed by providing a circularly polarized antenna on the interrogator and a dipole antenna on the tag.