In this chapter we study the building principles of the photosynthetic reaction centers that allow them to capture excitation energy from the antenna and to convert it into free energy of long-lived transmembrane charge-separated states. We will discuss standard theories of electron transfer and their extensions and how the parameters of the theories can be obtained from independent experiments and structure-based simulations. A simple derivation is presented for the semiclassical Marcus rate constant for nonadiabatic electron transfer. The derivation is based on a normal mode description of the pigment–protein complex, a linear electron–vibrational coupling, the potential energy difference between the reactant and product states as a generalized reaction coordinate, and the central limit theorem of statistical mechanics. Next, we derive quantum corrections to this rate constant and obtain a simple explanation why these effects are particularly important in the inverted region of electron transfer. We review calculation schemes for the parameters of the theory and relate the molecular structures of the reaction centers to their function. Attention is paid to the question of what are the common building principles that have led to a very similar structure of all known reaction centers and how subtle differences are related to an optimization of photochemical efficiency. As an important example of such an optimization, we compare the type II reaction centers of photosystem II and purple bacteria. We argue that nature had to change some mechanistic details of the primary reactions to allow photosystem II to use water as an electron source.