Fluorescence microscopy requires high photon-flux densities in the specimen plane. These intensities are only achieved by lasers, arc lamps, and, most recently, light-emitting diodes (LEDs). Lasers and LEDs, however, are restricted to a limited number of wavelength regions, whereas with arc lamps it is possible to select arbitrary wavelengths and wavelength regions. Moreover, the lower cost of arc lamps, compared with lasers, makes them the light source of choice for the majority of fluorescence microscopy applications. Recently, so-called white light lasers have become commercially available, but their photon fluxes--although sufficient for laser scanning applications--are still not high enough for applications where extended areas need to be illuminated. This article discusses arc lamps and the design and performance of an arc lamp-based illumination system for fluorescence microscopy that allows the user to choose any wavelength from ultraviolet (UV) to infrared. The system permits rapid switching speed between colors, while maintaining quite stable and homogenous emissions.