These notes offer a comprehensive discussion of the experimental and theoretical work done so far on the F' centers in alkali halides. Here the F' center is a two-electron single-anion-vacancy entity with no other anion vacancies in its immediate neighbourhood. This definition allows for the inclusion of pair interactions between color center species which occupy a central position in the book. Quantities that control or characterize F'-related physics, such as the F-F' conversion efficiency at low temperatures or high excitation levels, the F' thermal lifetime, the F' bandshape, are given a...