As the name “ad hoc” indicates, an ad hoc network is a wireless network designed to satisfy a “special purpose” application, often of short duration. An ad hoc network must be deployed rapidly and is frequently reconfigured to adjust to changes. The temporary nature and the high rate of change set the ad hoc network apart from other more permanent wireless structures such as urban mesh networks or environmental sensor networks. A prime example of an ad hoc environment is the battlefield. It is no surprise that much of the early work on ad hoc nets was supported by the Department of Defense, through the DARPA Packet Radio program that began just a couple of years after the introduction of the ARPANET. Civilian applications (natural disaster management, homeland defense, etc.) have followed closely the military model and in fact share many of its requirements. Commercial applications have just begun to emerge, in most cases as “opportunistic extensions” of the network infrastructure (wired or wireless). One promising commercial application is car-to-car ad hoc networking. The vehicular network application is in part propelled by the Department of Transportation plan to develop a “standard” vehicular radio architecture — DSRC (Digital Short Range Communications) — for navigation safety. Once the radio is installed on each car for a civilian purpose (e.g., safe driving), one can speculate that a number of commercial applications will rapidly spring up. Most of the vehicle applications will revolve around wireless Internet access; but, some will be based on fully distributed applications, e.g., car-to-car file sharing and games. Besides vehicular ad hoc networking, other commercial applications are now emerging which include multihop extensions of the urban “mesh networks” to mobile momentarily out of reach. This trend is fueled by the proliferation of IEEE 802.11 wireless LAN products and services together with development of ad hoc protocol standards in IETF MANET.