Photovoltaic power conversion efficiency is the single most important performance indicator. This paper gives an overview of procedures to determine the efficiency with respect to standard conditions. The measurement theory and general procedures for determining the efficiency with respect to reference conditions are well understood. The engineering challenge of commercial or custom equipment to perform accurate efficiency measurements for all photovoltaic technologies is discussed. Accurately measuring the performance of multijunction devices requires adjusting the intensity and spectral content of the light source so that each junction generates the proper photocurrent. Organic devices often have time constants on the photocurrent in excess of 1 s. This poses a variety of problems when measuring the quantum efficiency under 1-sun illumination or measuring the current-voltage characteristics. Many photovoltaic cells have reversible and irreversible transients in the current and/or voltage. Module acceptance testing and qualification procedures often assume that these transients are not present. The sample with the highest efficiency may not produce the highest energy because the photovoltaic performance is a function of the spectral content of the light, total irradiance, and temperature-all of which are different in the "real world" than under standard reference conditions.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.