This paper provides an easy-to-use screening methodology to estimate potential excess lifetime lung cancer risk resulting from indoor radon exposure. The methodology was developed under U.S. EPA Office of Policy, Planning, and Evaluation sponsorship of the agency's Integrated Environmental Management Projects (IEMP) and State/Regional Comparative Risk Projects. These projects help policymakers understand and use scientific data to develop environmental problem-solving strategies. This research presents the risk assessment methodology, discusses its basis, and identifies appropriate applications. The paper also identifies assumptions built into the methodology and qualitatively addresses methodological uncertainties, the direction in which these uncertainties could bias analyses, and their relative importance. The methodology draws from several sources, including risk assessment formulations developed by the U.S. EPA's Office of Radiation Programs, the EPA's Integrated Environmental Management Project (Denver), the International Commission on Radiological Protection, and the National Institute for Occupational Safety and Health. When constructed as a spreadsheet program, the methodology easily facilitates analyses and sensitivity studies (the paper includes several sensitivity study options). The methodology will be most helpful to those who need to make decisions concerning radon testing, public education, and exposure prevention and mitigation programs.