In the first part of this review paper, secondary processes in the formation of F centers are discussed : recombination of interstitials, VK centers and vacancies between themselves, decreasing the number of apparent F centers. In a second part, recent results on the primary process are exposed : why an excitonic recombination forms an F-H center pair, why the H center is ejected far enough to have only a small probability to recombine with its associate vacancy. In the last part, the formation mechanism of F aggregate centers is discussed