We present an overview of the physics and practical issues related to surface photovoltage measurement of the minority carrier diffusion length and its application to monitoring recombination center defects in silicon. A tutorial description is given of the role of pertinent processes like injection, recombination and trapping. The evolution of the SPV method is presented followed by a description of most recent refinements addressing the measurement of long diffusion lengths in silicon wafers with emphasis on accuracy, measuring speed, tool-to-tool reproducibility and, practically the most important question of monitoring, iron concentration.