This chapter presents an overview of structural properties of the cytochrome (Cyt) b f complex and its functioning in chloroplasts. The Cyt b f complex stands at the crossroad of photosynthetic electron transport pathways, providing connectivity between Photosystem (PSI) and Photosysten II (PSII) and pumping protons across the membrane into the thylakoid lumen. After a brief review of the chloroplast electron transport chain, the consideration is focused on the structural organization of the Cyt b f complex and its interaction with plastoquinol (PQH, reduced form of plastoquinone), a mediator of electron transfer from PSII to the Cyt b f complex. The processes of PQH oxidation by the Cyt b f complex have been considered within the framework of the Mitchell's Q-cycle. The overall rate of the intersystem electron transport is determined by PQH turnover at the quinone-binding site Q of the Cyt b f complex. The rate of PQH oxidation is controlled by the intrathylakoid pH, which value determines the protonation/deprotonation events in the Q-center. Two other regulatory mechanisms associated with the Cyt b f complex are briefly overviewed: (i) redistribution of electron fluxes between alternative (linear and cyclic) pathways, and (ii) "state transitions" related to redistribution of solar energy between PSI and PSII.