The western boundary currents, such as the Gulf Stream and Kuroshio Extension, are highly energetic currents that transport heat poleward from the tropics. Quantifying their heat transport is difficult due in large part to sparse observations and the turbulent nature of the flow field. The most energetic scales in these currents are due to mesoscale eddies, which have length scales of O(100 km) and time scale of days to months. The Kuroshio Extension System Study (KESS) was a two-year observational field program to measure for the first time the mesoscale eddying 4-D current and density field east of Japan. This talk will present some of the results from the KESS field program and compare the results with a state-of-the-art high-resolution ocean model.