Water is a core infrastructure sector that serves communities and businesses on a daily basis. Clean drinking water sustains core functions of our society which makes safety and security of water infrastructure a top priority. The objective of the ERNCIP Chemical and Biological Risks to Drinking Water Thematic Group (TG Water) is to strengthen resilience of drinking water infrastructures against Chemical and Biological (CB) waterborne threats by improvement of detection capabilities and early response actions. The implementation of security measures to counter hostile actions against the physical and cyber integrity of water supply systems and deliberate waterborne contamination requires an appropriate planning process incorporating risk assessment surveys, establishment of communication strategies, protocols and screening methods. Water security planning helps to identify security vulnerabilities and establish security measures to detect the intentional contamination of water supply systems, including a communication strategy to facilitate a fast and effective response. Where a water safety plan already exists (European Commission 2015), water security planning should be integrated into the safety planning approach. This manual supports the development of a separate stand-alone water security plan in cases where a water safety plan is not yet in place and encourages a complementary approach in cases where there are existing water safety plans, including the consideration of an integrated approach where relevant. This manual aims to disseminate knowledge on how to implement a Water Security Plan, and complements the "Guidance on the production of a water security plan for drinking water supply" published in 2019 by the TG Water. The manual provides a detailed basis for the creation and implementation of a Water Security Plan for drinking water systems, supporting water utility operators with the information and tools they need to develop a plan specifically for the security of their water supply systems. An effective Water Security Plan, together with a continuously managed implementation, supports the optimization of equipment placement and resource allocation - either of human or economic nature - along the water supply system, as well as increasing the confidence level of water utilities to be able to cope with chemical/biological contamination.