The authors empirically test how 12th-grade students of teachers with probationary certification, emergency certification, private school certification, or no certification in their subject area compare relative to students of teachers who have standard certification in their subject area. The authors also determine whether specific state-by-state differences in teacher licensure requirements systematically affect student achievement. In mathematics, the authors find teachers who have a standard certification have a statistically significant positive impact on student test scores relative to teachers who either hold private school certification or are not certified in their subject area. Contrary to conventional wisdom, mathematics and science students who have teachers with emergency credentials do no worse than students whose teachers have standard teaching credentials.