Understanding the location and potential of renewable energy resources is a crucial pre-requisite to their utilization, and to scaling up clean and secure sources of electricity generation such as biomass, small hydropower, solar, and wind. However many countries do not have high quality, publicly available data on renewable energy resource potential and this limits the potential for informed policy development, including zoning guidance, transmission network planning, and price regulation or incentives. It also narrows the field of potential commercial developers, and raises the cost of undertaking preliminary site identification and financial analyses. This report draws on many years of experience within the World Bank Group and among other development partners in carrying out renewable energy resource assessment and mapping at the country level, in particular from 12 projects funded by the Energy Sector Management Assistance Program (ESMAP) under a major global initiative launched in 2012. The report's purpose is to explain, for a wide range of audiences, the importance of resource assessment and mapping, key steps and good practices, methodological issues, and potential sources for further advice and support.