Semiconductor nanocrystal colloids are most striking for the ease with which their color, determined by electronic absorption frequencies, can be controlled by size. Although most of the applications currently envisioned are based on the interband transitions, one should not overlook the intraband transitions. In the case of conduction band states, these transitions are easily size-tunable through spectral regions of atmospheric transparencies (e.g., 3–5 µm and 8–10 µm). This tunability makes semiconductor nanocrystal colloids attractive subjects of study with potential applications in filters, detectors, lasers, and nonlinear optical elements.