Having discussed some of the fundamental aspects of the ocean hydrologic cycle and its importance to ocean dynamics, it is now useful to identify approaches with which we will be able to improve open ocean estimates of water exchange with the atmosphere. As there is no ``reference standard'' against which to calibrate such estimates, we will be best served by a multitude of approaches. Both the mean state and its temporal variability are of interest; the different techniques possess strengths and weaknesses for these applications.