Complex electro-optical (E-O) systems may be integrally mounted within the airframe or housed in separate pods on aircraft. Sensor fields of view and integrated laser target designator/rangefinder beams commonly sweep over large solid angles. This is accomplished by gimbaled mounts that are normally line-of-sight stabilized. If the E-O system is gimbaled as a whole, simple flat windows suffice. Internally gimbaled systems require domes or multi- segmented (faceted) windows. In any event, windows form the environmental barrier to the outside world for the E-O system and often must accommodate spectra other than just infrared (FLIR) wavebands. Optical windows can easily be the single most expensive optical component of a system. This paper considers various window designs and discusses their unique properties and possible pitfalls. Besides optical properties--durability, thermal, electromagnetic shielding, and other environmental aspects are treated.