In reaction centers, the physics of light harvesting gives way to the chemistry of charge separation. The electron flow initiated by the delivery of excitation energy from antenna pigment proteins to the reaction center enables sunlight to power the synthesis of key molecules such as ATP and NADPH, leading to cell growth and division. This chapter provides a biochemist’s perspective on the structures of the three best characterized reaction centers, what is known about the mechanisms through which harvested light energy triggers charge separation, and why variations on the common structural and mechanistic theme have arisen and how they are enabled.