The photosystem II complex, which is the most abundant membrane protein in chloroplasts, comprises the light-harvesting complex II and a reaction-centre core. The reaction centre uses the solar energy collected by the light-harvesting complex II to withdraw electrons from water, releasing oxygen into the atmosphere, It thus generates an electrochemical potential, providing the energy for carbon dioxide fixation and the synthesis of organic molecules, which make up the hulk of the biosphere(1). The structure of the light-harvesting complex II has been determined at 3.4-Angstrom resolution by electron crystallography(2), but the high-resolution structure of the photosystem II reaction centre and other core components remained unknown. We have grown well-ordered two-dimensional crystals of a sub-core complex containing the reaction centre from spinach thylakoid membranes and used electron crystallography to obtain a projection map of its structure at 8-Angstrom resolution. The features reveal the likely location of the key components that are active in electron transport, and suggest a structural homology and evolutionary links, not only with the purple bacterial reaction centre but also with the reaction centre of photosystem I.