This study seeks to empirically ground the debate over mathematics/science teacher shortages, and evaluate the extent to which there is, or is not, a sufficient supply of teachers in these fields. Our analyses of nationally representative data from multiple sources show that mathematics and science are the most difficult-to-staff fields, but the factors behind these problems are complex. There are multiple sources of new teachers; those with education degrees are a minor source compared to those with degrees in mathematics and science, and the reserve pool. Over the past two decades, graduation requirements, student course taking, and teacher retirements have all increased for mathematics and science, yet the new supply has more than kept pace. However, when preretirement teacher attrition is factored in, there is a much tighter balance between supply and demand. Unlike fields such as English, for mathematics/science there is not a large cushion of new supply relative to losses—resulting in staffing problems in schools with higher turnover.