In this article, the authors explore the level, variation, and change in teacher knowledge and instruction in the first two years of teaching, the relationship between Mathematical Knowledge for Teaching (MKT) and more distal measures such as certification. The findings reveal that many beginning math teachers had neither a degree in math nor substantial coursework in math. The authors also found that beginning teachers in this study generally had low levels of knowledge (as measured by the MKT), a balanced approach to cognitive demands, low levels of discussion quality, and substantial across-teacher variation in topic coverage. Furthermore, this study provides empirical evidence documenting that in their first two years of teaching, middle school math teachers improved in their math knowledge and improved on some but not all measures of instructional quality.