The "fundamental parameters" approach to calibration in X-ray fluorescence is unique because it is based upon the theoretical relationship between measured X-ray intensities and the concentrations of elements in the sample. This theoretical relationship is based upon X-ray physics and the measured values of fundamental atomic parameters in the X-ray region of the electromagnetic spectrum. In this tutorial, an introduction to the means of calibration is provided based upon a simplified instrument–sample geometry, thus eliminating some of the mathematical details of the traditional derivations.