This essay, authored by Keith Beven from Lancaster University, UK, discusses the concept of equifinality in hydrological modeling and its implications for model evaluation and uncertainty estimation. The author argues that traditional calibration methods are incomplete and that multiple acceptable models can represent hydrological and environmental systems, a concept known as equifinality. Beven proposes an extended Generalized Likelihood Uncertainty Estimation (GLUE) methodology to address these issues more rigorously. The essay explores the philosophical underpinnings of equifinality, the challenges of model evaluation, and the need to consider multiple sources of error in hydrological models. It also discusses the limitations of additive error models and the importance of deconstructing model error to better understand its components. The essay concludes with a proposal for a future research program that embraces equifinality, emphasizing the need to consider multiple feasible models and their uncertainties in hydrological modeling.This essay, authored by Keith Beven from Lancaster University, UK, discusses the concept of equifinality in hydrological modeling and its implications for model evaluation and uncertainty estimation. The author argues that traditional calibration methods are incomplete and that multiple acceptable models can represent hydrological and environmental systems, a concept known as equifinality. Beven proposes an extended Generalized Likelihood Uncertainty Estimation (GLUE) methodology to address these issues more rigorously. The essay explores the philosophical underpinnings of equifinality, the challenges of model evaluation, and the need to consider multiple sources of error in hydrological models. It also discusses the limitations of additive error models and the importance of deconstructing model error to better understand its components. The essay concludes with a proposal for a future research program that embraces equifinality, emphasizing the need to consider multiple feasible models and their uncertainties in hydrological modeling.