Correctly measuring radiometric quantities and reporting radiometric data is essential in developing a more complete understanding of the requirements and polymerization kinetics of dental resin composites. Presented herein is an introduction to the fundamental concepts, quantities and units of radiometry. A brief discussion of photometry and how it differs from radiometry is also included. While the basic principles of radiometry are relatively straightforward, a great deal of attention must be paid to the details when performing radiometric measurements and reporting radiometric data. Meaningful radiometric measurements require very close attention to nomenclature and mathematical definitions. Radiometric, not photometric quantities should always be used, unless the goal is to report how the human eye perceives light.