Education researchers and policy makers agree that teachers differ in terms of quality and that quality matters for student achievement. Despite prodigious amounts of research, however, debate still persists about the causal relationship between specific teacher credentials and student achievement. In this paper, we use a rich administrative data set from North Carolina to explore a range of questions related to the relationship between teacher characteristics and credentials on the one hand and student achievement on the other. Though the basic questions underlying this research are not new - and, indeed, have been explored in many papers over the years within the rubric of the "education production function" - the availability of data on all teachers and students in North Carolina over a ten-year period allows us to explore them in more detail and with far more confidence than has been possible in previous studies. We conclude that a teacher's experience, test scores and regular licensure all have positive effects on student achievement, with larger effects for math than for reading. Taken together the various teacher credentials exhibit quite large effects on math achievement, whether compared to the effects of changes in class size or to the socio-economics characteristics of students, as measured, for example, by the education level of their parents.