Hitherto, two families of multielement detectors have been used for IR applications: scanning systems (first generation) and staring systems (second generation). Third-generation systems are being developed nowadays. In the common understanding, third-generation IR systems provide enhanced capabilities-like larger numbers of pixels, higher frame rates, and better thermal resolution as well as multicolor functionality and other on-chip functions. In the paper, issues associated with the development and exploitation of third-generation IR photon detectors are discussed. In this class of detectors two main competitors, HgCdTe photodiodes and quantum-well photoconductors, are considered. The main challenges facing multicolor devices concern complicated device structures, thicker and multilayer material growth, and more difficult device fabrication, especially when the array size gets larger and pixel size gets smaller. Also, technical developments that are key to third-generation devices, such as vapor phase epitaxy and advanced readout concepts, are discussed.