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 due space. A number of experimental situations related to F' centers are also discussed. Finally, a recently revived attempt to deal with the F-F'conversion in terms of the electron transfer via (bound) polarons, a work still in progress, is described. These notes are expected to stimulate further experimental and theoretical investigations on the F' centers.