Diffraction gratings are the most important wavelength dispersing optic from the soft x-ray to the infrared. This tutorial outlines the important details about the manufacture and specification of this important optical component. Both ruled and interferometrically generated gratings are covered, emphasizing the differences and unique advantages of each type. Important applications of gratings are briefly covered at the end of the tutorial. The tutorial was presented at the SPIE Annual Meeting in San Diego in conjunction with Conference 503, "International Conference on the Application, Theory, and Fabrication of Periodic Structures, Diffraction Gratings, and Moire Phenomena II.