To stabilize the climate, global greenhouse gas emissions must be cut by 25 to 50 percent by 2030 compared to 2019. Such an unprecedented rate of decarbonization necessitates climate mitigation policies across countries, notably carbon pricing, fossil fuel subsidy reform, renewable subsidies, feebates, emission rate regulations, and public investments. To design and implement effective, efficient, and equitable policies, governments need tools to assess economic, environmental, fiscal, and social impacts. To support this effort, the IMF and World Bank are making their joint Climate Policy Assessment Tool (CPAT) available to governments. CPAT is a transparent, flexible, and user-friendly model covering over 200 countries. It allows for the rapid quantification of impacts of climate mitigation policies, including on energy demand, prices, emissions, revenues, welfare, GDP, households and industries, local air pollution and health, and many other metrics. This paper describes the CPAT model, its data sources, key assumptions, and caveats.