Bluetooth (Bluetooth SIG, 2004) and ZigBee (ZigBee Alliance, 2004) are short-range radio technologies designed for wireless personal area networks (WPANs), where the devices must have low power consumption and require little infrastructure to operate, or none at all. These devices will enable many applications of mobile and pervasive computing. Bluetooth is the IEEE 802.15.1 (2002) standard and focuses on cable replacement for consumer devices and voice applications for medium data rate networks. ZigBee is the IEEE 802.15.4 (2003) standard for low data rate networks for sensors and control devices. The IEEE defines only the physical (PHY) and medium access control (MAC) layers of the standards (Baker, 2005). Both standards have alliances formed by different companies that develop the specifications for the other layers, such as network, link, security, and application. Although designed for different applications, there exists some overlap among these technologies, which are both competitive and complementary. This article makes a comparison of the two standards, addressing the differences, similarities, and coexistence issues. Some research challenges are described, such as quality of service, security, energy-saving methods and protocols for network formation, routing, and scheduling.