Quantum Well Infrared Photodetectors (QWIPs) offer greater flexibility than usual extrinsically doped semiconductor IR detectors because the wavelength of the peak response and cutoff can be continuously tailored by varying layer thickness (well width), barrier composition (barrier height), and carrier density (well doping density). The GaAs/AlxGa1- xAs material system allows the quantum well parameters to be varied over a range wide enough to enable light detection at any wavelength range between 6 - 20 micrometer. The spectral band width of these detectors can be tuned from narrow ((Delta) (lambda) /(lambda) approximately 10%) to wide ((Delta) (lambda) /(lambda) approximately 40%), allowing various applications. Also, QWIP device parameters can be optimized to achieve extremely high performance at lower operating temperatures (approximately 40 K) for low background, long- wavelength, infrared applications in the strategic arena as well as in Astronomy. Furthermore, QWIPs offer low cost per pixel and highly uniform, large format, focal plane arrays (FPAs) mainly due to mature GaAs/AlGaAs growth and processing technologies.