This Technical Memorandum addresses visibility-related spectral effects of lighting at mesopic light levels. There is a great deal of interest in the impact of light sources with differing spectral power distributions in terms of visibility at the relatively low light levels used for much outdoor and roadway lighting. The purpose of this document is to provide the reader with a summary of the research in this area, as well as to the potential approaches and caveats involved in incorporating this information into lighting practice.