Strong, persistent currents along the western boundaries of the world's major ocean basins are called "western boundary currents" (WBCs). This chapter describes the structure and dynamics of WBCs, their roles in basin-scale circulation, regional variability, and their influence on atmosphere and climate. WBCs are largely a manifestation of wind-driven circulation; they compensate the meridional Sverdrup transport induced by the winds over the ocean interior. Some WBCs also play a role in the global thermohaline circulation, through inter-gyre and inter-basin water exchanges. After separation from the boundary, most WBCs have zonal extensions, which exhibit high eddy kinetic energy due to flow instabilities, and large surface fluxes of heat and carbon dioxide. The WBCs described here in detail are the Gulf Stream, Brazil and Malvinas Currents in the Atlantic, the Somali and Agulhas Currents in the Indian, and the Kuroshio and East Australian Current in the Pacific Ocean.