This chapter first summarises different approaches to hydrological and hydrochemical modelling of freshwater, and then goes on to provide an overview of how models are being used in practice to address typical policy and environmental issues, with a special focus on Nature-based Solutions (NBS). For a broader overview of hydrologic and water quality models the reader is referred to review articles such as Singh and Woolhiser (2002), Borah and Bera (2003), Cox (2003), Kampf and Burges (2007), Schoumans et al. (2009a, 2009b), Arthington et al. (2010), Ampadu et al. (2013), Kelly et al (2013), Li and Heap (2014). Several categories of NBS approaches, as defined by Cohen-Schacham et al. (2016) relate to the water environment. These include: climate adaptation services (e.g. modification of water use through changed agricultural practice), natural/green infrastructure (e.g. natural flood retention ponds), integrated water resources management to meet the needs of multiple stakeholders, and area-based conservation (e.g. protection of wetland areas). These approaches are not specifically new, but can be clustered as part of the overarching NBS concept and contribute to the development of an operational framework for NBS. This chapter considers the role of modelling in evaluating and designing NBS for water.