Stealth technology, otherwise called anti-detection technology, incorporates both design and materials engineered to make aircraft, missiles, and even ships difficult to see, hear, or otherwise detect electronically. For an airplane to achieve stealth, however, compromises must be made. These include reduced speed and maneuverability, not to mention higher design and operating costs.
For fighter planes in particular, speed is a primary asset. For maximum stealth, however, too much speed is a liability. This is because traveling at supersonic speeds, as most fighters do, creates a tremendous amount of noise, what with the powerful engines expelling air and the sonic boom made when a plane accelerates above the speed of sound. Though some pilots would gladly trade some speed for enhanced stealth, others, whose missions may involve air-to-air combat, would opt for speed at the expense of some quietness.
Certain detection and missile systems home in on heat emitted by aircraft to detect their presence. Relocating the engine intake and exhaust equipment to above the wings can lessen the possibility for detection from below. Doing so, however, detracts from overall maneuverability.
Radar systems track airplanes by sending out bursts of electromagnetic waves that, on encountering an object, get reflected back to their antennae. Depending on how much of the wave energy is returned and how fast it returns, a radar operator can be alerted to the relative size and location of the object. Several "hits" on the object can also tell the operator in which direction and how fast a moving object is traveling. To make tactical military jets less visible to radar, they must be carefully designed so that any radar waves that are reflected from their surfaces do not return to the transmitter-receiver.