The cytochrome b(6) f (cytb(6) f) complex has a central role in oxygenic photosynthesis, linking electron transfer between photosystems I and II and converting solar energy into a transmembrane proton gradient for ATP synthesis(1-3). Electron transfer within cytb(6) f occurs via the quinol (Q) cycle, which catalyses the oxidation of plastoquinol (PQH(2)) and the reduction of both plastocyanin (PC) and plastoquinone (PQ) at two separate sites via electron bifurcation(2). In higher plants, cytb(6) f also acts as a redoxsensing hub, pivotal to the regulation of light harvesting and cyclic electron transfer that protect against metabolic and environmental stresses(3). Here we present a 3.6 angstrom resolution cryo-electron microscopy (cryo-EM) structure of the dimeric cytb(6) f complex from spinach, which reveals the structural basis for operation of the Q cycle and its redox-sensing function. The complex contains up to three natively bound PQ molecules. The first, PQ1, is located in one cytb(6) f monomer near the PQ oxidation site (Q(p)) adjacent to haem b(p) and chlorophyll a. Two conformations of the chlorophyll a phytyl tail were resolved, one that prevents access to the Qp site and another that permits it, supporting a gating function for the chlorophyll a involved in redox sensing. PQ(2) straddles the intermonomer cavity, partially obstructing the PQ reduction site (Q(n)) on the PQ1 side and committing the electron transfer network to turnover at the occupied Q(n) site in the neighbouring monomer. A conformational switch involving the haem c(n) propionate promotes two-electron, two-proton reduction at the Q(n) site and avoids formation of the reactive intermediate semiquinone. The location of a tentatively assigned third PQ molecule is consistent with a transition between the Q(p) and Q(n) sites in opposite monomers during the Q cycle. The spinach cytb(6) f structure therefore provides new insights into how the complex fulfils its catalytic and regulatory roles in photosynthesis.