Teacher production has important implications for our education system. Leveraging more than a decade of national data, the authors examine the supply of teachers and changes in the reserve pool of teachers in the United States. They find a substantial decrease in the teacher supply coinciding with the increase in demand for teachers starting in 2016, shifting the reliance on the reserve pool of teachers. By 2018, there was a shortfall of new teachers, and about 150,000 teachers were drawn from the reserve pool to meet demands. Without an increase in the supply of new teachers, the reserve pool will need to be relied on to meet demand, which may not be sustainable.