 The paper discusses historical controversies surrounding process-based hydrologic modeling due to challenges in defining suitable model equations, adequate parameters, and computational constraints. The authors provide examples of advances made by different groups using various types and complexities of models, arguing for the need to effectively utilize our diversity of modeling approaches to advance the quest for physically realistic hydrologic models. This article was authored by M. P. Clark, M. F. P. Birkins, Al-Samaniego, and others.