Rapid progress in the development of quantum well IR photodetectors (QWIPs) has made it possible to manufacture sensitive, large area, highly uniform GaAs/AlGaAs staring focal plane arrays (FPAs). Because of the maturity of the GaAs growth technology, the stability of the material system, and good performance in the long wave IR spectral region, QWIP technology development is supported by the Ballistic Missile Defense Organization. In comparing Mercury Cadmium Telluride (MCT) to QWIPs it is found that QWIPs are comparable or superior to MCT at 8 to 12 microns for some applications and can be used at wavelength in excess of 12 microns.QWIPs have greater flexibility in spectral range and offer the possibility of a superior multicolor capability using a single growth and fabrication processing technique. This makes simultaneous detection of two and more colors easier to achieve than in MCT. On the other hand, QWIP sensitivity is currently below that of MCT. Also MCT arrays can operate at higher temperatures with lower dark current than QWIPs. However, QWIPs produce a superior image quality due to better spatial and spectral uniformity. For example, high pixel to pixel uniformity allows QWIPs to perform well in interceptors where window heating generates a high thermal background, which must be accurately subtracted. This paper reviews the status of QWIP technology and presents several potential applications for QWIP sensors. All of these applications are for ballistic missile defense.