The IEEE 802.15.4 is a standard wireless personal area network defined by the IEEE 802.15 working group. The model was ratified in 2003 and forms the basis of many other protocols including Thread (covered later), Zigbee (covered later), WirelessHART, and others. 802.15.4 only defines the bottom portion (PHY and data link layer) of the stack and not the upper layers. It is up to other consortiums and working groups to build a full network solution. The goal of 802.15.4 and the protocols that sit on it are low-cost WPAN with low power consumption. The latest specification is the IEEE 802.15.4e specification ratified on February 6, 2012, which is the version we will discuss in this chapter.