Nature-Based Solutions and Water Security: An Action Agenda for the 21st Century presents an action agenda for natural infrastructure on topics of standards and principles, technical evaluation and design tools, capacity building and innovative finance. Chapters introduce the topic and concepts of natural infrastructure, or nature-based solutions (NBS) and water security, with important background on the urgency of the global water crisis and the role that NBS can, and should play, in addressing this crisis. Sections also present the community of practice's collective thinking on a prioritized action agenda to guide more rapid progress in mainstreaming NBS. With contributions from global authors, including key individuals and organizations active in developing NBS solutions, users will also find important conclusions and recommendations, thus presenting a collaboratively developed, consensus roadmap to scaling NBS.