Photosynthetic organisms that do not evolve oxygen are found in four of the 24 phyla of bacteria: Proteobacteria (purple bacteria), Chloroflexi (green, non-sulfur bacteria), Chlorobi (green, sulfur bacteria) and Firmicutes (Heliobacteria). The reaction centers (RCs) of photosynthetic Chlorobi and Firmicutes (Type I reaction centers) are structurally and functionally similar to the RC of Photosystem I in oxygenic organisms, while those of Proteobacteria and Chloroflexi (Type II reaction centers) resemble Photosystem II. Reaction centers of Type I transfer electrons from a c-type cytochrome or plastocyanin to bound [4Fe-4S] iron-sulfur centers, which then reduce the soluble iron-sulfur protein ferredoxin. Type II RCs transfer electrons from c-type cytochromes to a quinone that picks up protons and dissociates from the RC as the fully reduced quinol. This chapter describes the patterns of electron transfer through the RCs of the four families of bacteria, with an emphasis on the relationships between function and structure. These functional patterns are reasonably clear in proteobacterial RCs, but much remains to be learned about them in some of the other families.