Explains the rating system for US municipal bonds and its effect on borrowing costs, reviews relevant research and provides a study of the factors affecting grading by rating agencies in Virginia using 1995 data. Explains the methodology and presents the results, which identify five significant determinants of favourable ratings. Shows that net interest costs are lower when other rates of interest are low, real estate taxes are high (though not excessive), total municipal debt levels are low and credit risks are low. Confirms that bond ratings capture additional information and that a drop in ratings will raise net interest costs substantially. Considers consistency with other research and the implications of the findings for participants in the municipal bond market.