Any electro-optical (EO) sensor operating in an environment in which there is a scarcity of information with which to discriminate one type of object from another needs to use all avenues available to it. For passive, nonimaging EO sensors, there are very few discriminants available; the spectral content of the radiation emitted by the object is one. The effective and efficient real-time use of this discriminant is, however, technologically difficult to implement. Recent studies have reviewed a wide range of possible techniques and have identified the relevant critical technologies involved. Thirty-two such concepts, listed here, have been developed over the years for a variety of applications both in the laboratory and in the field. Most of these are too cumbersome, too complex, or too slow to operate reliably and in real-time stressing environments. Two near-term techniques are multiple fixed filters and exchangeable edge filters. Two mid-term techniques are miniature dual-tunable Fabry-Perot (DTFP) devices and slit spectrometers. Two advanced techniques that use emerging technologies are electrically tunable detector materials and integrated-optics interferometers.