 Hydrological models are used to simulate the behavior of water in the environment. Historically, there has been much debate about which approach to process-based hydrologic modeling is most appropriate. This paper reviews three main issues related to modeling, defining suitable model equations, defining adequate model parameters, and coping with limitations in computing power. Examples of modeling advancements that address these challenges are provided, as well as suggestions for future research needs. It is argued that the diversity of modeling approaches can be leveraged to advance our understanding of physical processes and ultimately lead to better hydrologic models. This article was authored by M. P. Clark, M. F. P. Birkins, L. Samani Yago, and others.