In October 2005 the Defense Advanced Research Projects Agency (DARPA) launched the Nano Air Vehicle (NAV) program with the objective of developing and demonstrating small (<10 cm wingspan), lightweight (<10 grams) air vehicle systems with the potential to perform challenging indoor and outdoor military missions. 1 The program was executed in two phases. The initial 18-month phase included four contractors-AeroVironment Inc., Charles Stark Draper Laboratory, Lockheed Martin Advanced Technology Laboratories, and MicroPropulsion Corp.-each creating a preliminary design of its NAV system through a combination of analysis, component testing, trade studies, and concept development.Technologies developed in Phase 1 included rotating and flapping wings for lift generation and vehicle control, algorithms for stability and control using video images, and analytical tools for low Reynolds number aircraft. In Phase 2 AeroVironment was selected to continue the development begun in Phase 1. Phase 2 concluded with a hummingbird-inspired prototype aircraft with two flapping wings that for the first time demonstrated precision hovering and fast forward flight using only the flapping wings for propulsion and control. This paper will describe the initial program goals, outline the technological challenges addressed, briefly describe the individual NAV projects and related technology programs, and provide insight into technical areas still in need of further development. I. NAV Program Motivation and GoalsThe military forces of the United States and its allies have an ever-present need for improved intelligence, surveillance, and reconnaissance (ISR). Detailed ISR on the ground and in urban environments is a particular highvalue challenge that can be viewed as a gap in current ISR capabilities. Warfighters currently lack a technology that can be readily and flexibly deployed as a mobile sensing system within buildings, around corners, over walls, and in other denied areas prior to entry. Appropriately instrumented nano air vehicles (NAVs), unmanned aerial vehicles (UAVs) approximately the size of large flying insects or small birds, could provide such mobile sensing capability, as well as persistent sensing through the delivery of small sensor packages. NAVs would expand and complement the current capabilities provided by ground (wheeled and tracked) robots and thrown sensors, because these devices are generally limited in mobility and perspective inside buildings and can be considerably larger and heavier than a NAV-sized aircraft.