More than half of all instrumental methods of analysis involve the measurement of either light absorption or light emission by a sample. For this reason, the analyst needs to have a basic understanding of the modern theory of light and parameters of light, namely wavelength, frequency, wave number, and energy, as well as the concept of how light interacts with matter and the theory behind the emission and absorption phenomena. This chapter introduces these concepts and continues with perhaps the oldest and some of the most popular of all instrumental analysis techniques, that of ultraviolet, visible, and infrared molecular absorption analysis. In fact, the molecular absorption spectrum is sometimes shown as a plot of absorptivity vs wavelength rather than absorbance vs wavelength. Fluorometry and absorption spectrophotometry are competing techniques in the sense that both are techniques for analyzing for molecular species.