Quantum well infrared photodetectors (QWIPs) form a new generation of infrared detectors based on carrier confinement in ultrathin semiconductor heterostructures. The artificial energy levels in these wells can be tailored to match any optical transition in the 3 - 20 micrometer photon wavelength range by adjusting the quantum well width and the barrier composition. In this communication, we summarize our present understanding of the physics of QWIP detection: photoexcited carrier emission and capture probability, contact injection, and noise mechanisms. We also present the performances of optimized devices for the infrared Detection in the 3 - 5 micrometer and 8 - 12 micrometer wavelength ranges. We also illustrate the major advantages of this new technology for infrared staring arrays: (1) standard III-V substrates and technology, thermal stability, uniformity, large areas, low development costs, radiation hardness, (2) adjustability from 3 to 20 micrometer, (3) new functions: multispectrality, spectrophotometry, band switching, optical reading.