The share of female teachers in the U.S. with an MA more than doubled between 1970 and 2000. This increase is puzzling, as it is much larger than that of other college-educated women, and it occurred over a period of declining teacher aptitude. I estimate the contribution of changes in teacher demographic characteristics, increases in the returns to an MA, and changes in teacher certification requirements to increases in teacher MA attainment rates. I find that the majority of the rise in attainment not attributable to secular trends and increases in the average age of teachers can be explained by increases in the returns to an MA among teachers. The increase in MA returns among teachers presents a second puzzle, as there is little evidence that master's degrees increase teacher productivity.