The optical constants of substances composing atmospheric aerosols are required to evaluate properly the effects of aerosols on the earth's radiation balance. We briefly review techniques for determining optical constants and also discuss pitfalls in using measured optical constants to simulate the optical constants of the real particles found in the atmosphere. We then compile the optical constants of (NH4)2SO4, Al2O3, and NaCl. The optical constants of Al2O3 and NaCl are derived from values quoted in the literature. The optical constants of (NH4)2SO4 are derived partly from values quoted in the literature and partly from our own measurements.