Boundary currents are critical elements in the global circulation and heat budget. Boundary currents carry mass, heat, freshwater and biogeochemical properties along their paths and into the ocean interior. The strength and variability of boundary currents affect the intensity of air-sea exchange of heat, moisture, and carbon and influence storm-track evolution and other extreme weather events. The global High Density eXpendable BathyThermographs (XBT) Network is an international effort that produces unique and high quality upper ocean temperature data across fixed repeat transects that, in conjunction with observations from other tools such as Argo floats and underwater gliders, monitor the variability of key ocean currents. XBTs monitor the variability of boundary currents, surface and subsurface currents, and basin-wide upper-ocean heat transport.