Groundwater is a vital common pool resource for water supply, irrigation, and ecosystems, but can be difficult to govern due to invisibility, conflicting interests, and limitations of available institutions. While there are many policy and technical instruments (tools) available, efforts to apply them are often ineffective. This special issue of the International Journal of the Commons presents a set of papers with insights into policy instruments and other methods for groundwater governance. The relevance and effectiveness of tools and combinations of tools (toolboxes) in addressing problems that emerge from groundwater use is related to how they fit with diverse physical and social contexts. Drawing on research and applied experience, including that presented in this issue, we outline a conceptual framework for groundwater governance that emphasizes attention not just to knowledge, but also to motivations, and to agency for effective coordination among key actors. Articles in the special issue analyze groundwater governance in areas of Africa (east, south, and north), Central Asia, India, and the United States. The articles cover a range of scales from small groups playing experiential games to international agreements about transboundary aquifers. Several papers illustrate the crucial role of knowledge about groundwater, and the need for governance instruments and interventions to go beyond only changing knowledge. Three papers focus on groundwater games and their use to understand and change behavior, especially when combined with other tools that facilitate collective deliberation and action. Several papers illustrate how understanding of the ways people care about and practice care for groundwater illuminates examples and capabilities for groundwater governance. Highlights There are many institutional tools for governing groundwater, but no panaceas, and successes are rare Effective groundwater governance requires that key stakeholders have combinations of knowledge, motivation, and agency to act together effectively Groundwater co-management can combine stakeholders’ knowledge, values, and collective action with external science, resources, and authority Participatory processes can craft combinations of tools to fit contexts and pursue shared gains