This chapter relates the structural features of photosynthetic reaction centres, discussed in the previous chapter, to their function, i.e., the capture of excitation energy, primary charge separation and charge stabilization. Special attention is paid to the question of what the common building principles are that have led to a very similar pigment organization in all known reaction centres of photosynthetic organisms, and how subtle differences in the structure are related to an optimization of photochemical efficiency. As an important example of such optimization we compare the type II reaction centres of purple bacteria and higher plants. Despite the very similar arrangement of chlorin pigments in the two reaction centres, there are important differences in the optical properties and of some mechanistic details of the primary photophysical reactions. For example, the sink for triplet and singlet excitation energy in the reaction centre of photosystem II of higher plants, and thereby the primary electron donor, is not the central chlorophyll dimer (the special pair) as in purple bacteria, but a special chlorophyll monomer. We argue that evolution had to incorporate these changes in order to allow photosystem II to use water as an electron source.