The Model Parameter Estimation Experiment (MOPEX) is an ongoing international project to help develop techniques for the a priori estimation of parameters used in land surface parameterization schemes of atmospheric and hydrological models. MOPEX is affiliated with both the Prediction in Ungauged Basins (PUBS) and the Global Energy and Water Cycle Experiment (GEWEX) and is supported by the GEWEX American Prediction Project (GAPP) as well as by individual participants. Current procedures for a priori parameter estimation are often based on relationships between model parameters and basin characteristics-that is, soils, vegetation, topography, climate, geology, etc. These developed relationships have not been fully validated through rigorous testing using retrospective hydrometeorological data and corresponding land surface characteristics. This is partly because the necessary database needed for such testing has not been available. Moreover, there still exists a gap in our understanding of the links between model parameters and land surface characteristics.