The many gaps in the relationship between theory and practice in hydrologic modeling and between hydrologic modeling and climate modeling are discussed. Important in connection with closing these gaps are topics such as the difficulty of estimating actual evaporation in the water balance at catchment and regional scale, the uncertainty about the role of vegetation in relation to changes in runoff following climate change, and the link that nonlinear system theory may provide between theoretical hydrology and climate theory. Present methods of communication between hydrologists and climate modelers will have to be improved if there is to be a fruitful dialogue and a synergistic relationship between the two groups. ¿ American Geophysical Union 1992