Critics of the rise in alternative and for-profit programs will claim teacher quality, and student learning, suffers when people are fast-tracked into the classroom without comprehensive training. But it’s hard to know for certain whether that’s true. The problem is, despite decades of trying, we haven’t agreed on how to measure teacher quality. There’s a lot of research that shows having a good teacher makes a huge difference in the outcomes of students, but it’s much less clear what makes a teacher good.