This paper presents a review of methodologies and results of evaluations examining hours-of-use in compact fluorescent lamps (CFLs). Hours-of-use is a crucial variable in determining the cost-effectiveness of lighting programs, and there is extensive variation in hours-of-use estimates across methodologies and within a particular methodology. Given the different sources of variation, one must be careful in comparing the results from different studies. However, we are able to recommend a robust methodology that should be used by evaluators of lighting programs, regardless of geography or program delivery type. (C) 2006 Elsevier B.V. All rights reserved.