Photosynthesis, the conversion of light energy into chemical energy, has been the basis of life on Earth for more than 3 billion years. The first steps of this conversion are light-driven electron transfer reactions catalyzed by the so-called photosynthetic reaction centers (RCs), which are complexes of protein subunits and pigments located in biological membranes. In this lecture I will present an overview of known three-dimensional structures of RCs from purple bacteria and from cyanobacteria and green plants (Photosystems I and II). Despite different sizes and compositions, the RCs 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.