Radio Frequency Identification (RFID) has been around since early 2000. Its use has currently become commonplace as the cost of RFID tags has rapidly decreased. RFID tags have also become more ‘intelligent’ with the incorporation of processors and sensors in them. They are widely used now in many innovative ways. RFIDs are an integral part of Internet of Things (IOT) and IT systems of smart cities. In this article, we introduce the technology used by RFID systems, illustrate their use in several applications, and discuss problems of privacy and security when they are used.