The use of analytical modelling tools in integrated water resources management (IWRM) provides important instruments both for finding the best water use solutions and achieving water security for multiple purposes in a sustainable and equitable manner. This Technical Focus Paper provides guidelines for elaborating and validating such modelling/Decision Support System tools to assist decision-makers in implementing IWRM. It is a joint knowledge product of GWP, UNEP-DHI Center, DHI and SIWI.