The goal of the paper is to review the various types of tools that are used for water resources assessment and planning efforts and to summarize the appropriate usage of these tools within a decision-making framework. It begins by defining what we mean by “tools” and then presenting general categories that are used in water planning efforts. A brief discussion of the time scale of water planning is presented to highlight how the urgency of decision-making influences the degree to which stakeholders may or may not be involved. The paper then walks through the steps of the WSI process and explains how various tools may be applied within this context. It concludes with an annex that provides illustrative examples of different tools used in water planning.