The conversion of light energy from the sun into chemical energy has been the basis of life on Earth for more than 3 billion years. The first steps of this process consist of the absorption of a photon, followed by electron transfer across a biological membrane. These reactions are catalyzed by complexes of proteins and pigments, called Photosynthetic Reaction Centers (PRCs). Studies using X-ray crystallography and cryo-electron microscopy, performed during the past four decades, have led to the determination of three-dimensional structures of PRCs from bacteria and green plants. Despite different sizes and compositions, the PRCs have at their cores arrangements of transmembrane helices and pigments that are amazingly similar. This supports the assumption that they all derive from a common ancestor.