This helpdesk report highlights some of the best practice examples of Nature-based Solutions (NbS) for water security and examines the implementation challenges and lessons learned. NbS is an umbrella term for a range of approaches and activities including source water protection, watershed management, wetlands restoration, protection, and construction, water harvesting, agricultural best management practices, afforestation, sustainable drainage systems and protecting mangroves, amongst others. Water security is important for sustaining livelihoods, human well-being and socio-economic development. Water insecurity is increasing and nature-based solutions (NbS) can address some key water security challenges. Best practice examples of nature-based solutions include: water for agriculture; source water protection and water funds and urban green infrastructure. Combining green and grey infrastructure can improve storage and supply, lower costs, produce more resilient services, enhance system performance and better protect communities. There is some evidence that green infrastructure performs equal or better than grey infrastructure and is cost-effective in comparison. Scaling-up NbS faces the following implementation challenges: finance; scale and context; equity; stakeholder engagement and gaps in technical guidance. There is a high degree of variation in how ecosystems impact on hydrology. Consequently, site-specific knowledge will be important in implementing NbS