Teachers do make a difference in student outcomes--and it can be a big one. Research has shown this to be especially true in mathematics, which forms the foundation for all future STEM learning. But most U.S. students do not get a series of good teachers--and highly skilled math and science teachers are not the norm. Excellent instruction requires both deep content knowledge and expertise in teaching that content to all kinds of learners. Developing inspiring STEM instruction is particularly demanding, while STEM teachers are particularly poorly prepared. This STEM Smart Brief describes the research and promising practices regarding teacher preparation, professional development, and school leadership strategies that are effective in improving teacher quality or student outcomes.