This chapter presents a deep dive into the current state of science and knowledge around water risks and water security as they relate to nature-based solutions (NBS). It explores the various ways that NBS affect water quantity, quality, and water-related hazards from the point of view of several types of key water users. It presents the strength of the evidence about these relationships and where gaps exist in this evidence base. Finally, this chapter introduces how this scientific knowledge is being put into practice through implementation of NBS and how robust monitoring and evaluation of on the ground projects can continue to build on this scientific understanding.