Electronic tagging can be divided into technologies that have the resources (active tag), and they are supported by the tag interrogation signal (passive tags), the focus here is on passive tags. An overview of the principles of passive tag divides technology into devices that use either near or far field coupling the field to communicate with tag readers. One part of the lecture has been dedicated to the principles of reading the tags co-located, as this is a challenge for a technology that may one day be able to automatically identify all the items in your shopping cart in a few seconds. Extension of the use of RFID read / write memory is integrated with the tag are also discussed, in particular saw a novel near-term opportunities. Finally, RFID is considered the remaining challenges and we see the possibility of future technology.