This lecture provides an introduction to Radio Frequency Identification (RFID), a technology enabling automatic identification of objects at a distance without requiring line-of-sight. Electronic tagging can be divided into technologies that have a power source (active tags), and those that are powered by the tag interrogation signal (passive tags); the focus here is on passive tags. An overview of the principles of the technology divides passive tags into devices that use either near field or far field coupling to communicate with a tag reader. The strengths and weaknesses of the approaches are considered, along with the standards that have been put in place by ISO and EPCGlobal to promote interoperability and the ubiquitous adoption of the technology. A section of the lecture has been dedicated to the principles of reading co-located tags, as this represents a significant challenge for a technology that may one day be able to automatically identify all of the items in your shopping...