_Active sensors:_ By far, the most important of the energy-emitting sensors is radar. Among the best all-weather capabilities of any type of sensor, the role for and capabilities of radar have steadily increased since the Second World War. Radar systems are used for early warning, air defense, air asset management, air traffic control, naval fleet defense, detection and tracking of moving ground targets, missile targeting, missile terminal guidance, terrain data development, and weather prediction. For Rapid Dominance, radars and other active sensors must operate with low probability of intercept. Particularly with stealthy systems, this will present a unique challenge to military systems where one may not expect a great amount of "spin-on" from the commercial sector. It is vitally important to be able to sense the enemy under all conditions and environments. Sensors must penetrate foliage and walls and detect threats such as underground and underwater mines. There are many other important active sensor classes, three of which are active acoustics, lidar and magnetic anomaly detectors. Broadband underwater active acoustics could address pressing needs such as shallow-water anti-submarine warfare and mine detection (both buried and silt covered). The practical application of lidar is a relatively recent development enabled by advances in laser, power management, and data processing technologies. Lidar can be used for fire control, weapon guidance, foliage penetration (vegetation is translucent in the near infrared (NIR) regime), and target imaging/recognition. Lidar detects shape directly and shape fluctuations such as vibration and motion and has proven very hard to spoof. Magnetic anomaly detectors will continue to find application in areas of anti-mine and anti-submarine warfare and in screening for weapons at security checkpoints and elsewhere. Electronic emissions are of themselves a liability only where they create a signature of use to an enemy. The ability to emit energy, yet in ways that are less discernible, should be an attractive avenue to explore for the future. The coordinated application of many sensor platforms, some of which may be completely passive, in conjunction with emitting sensors is a potentially major area of exploration. _Passive sensors:_ Among the passive sensor types, the most important for U.S. forces is forward-looking infrared (FLIR). FLIR technology has allowed the U.S. to "own the night," as was handily displayed in _Operation Desert Storm_. Some of the significant technology advancements underway in this area include multiple wavelength sensors, very large focal planes, and the increasing performance of uncooled sensors. Particularly in the area of uncooled sensors, commercial developments are underway that promise to drastically reduce the cost of competent IR sensors.