Since its introduction by educational psycholigist Lee Shulman in the late 1980s, the term "pedagogical content knowledge" (PCK) has been used often to refer to the particular knowledge needed by teachers to effectively create learning environments that support student sense-making in mathematics (and other disciplines). However, until recently there has been no good way to measure a teacher's PCK.
This is no longer the case and we now need to learn from the work that has led to greater insights into PCK among teachers of mathematics. With the recent work of U. S. mathematics education researchers Heather Hill and Deborah Ball some progress has been made with this at the elementary and middle school levels and researchers led by Jurgen Baumert in Germany have helped to capture PCK among secondary teachers of matheamtics. The bottom line is that while content knowledge is clearly necessary for teachers of mathematics, it is not sufficient. The most effective teachers also have insights into the mathematics content their students will learn that allows them to identify misconceptions, set up lessons in which students make connections, and provide students with questions and prompts that help them access important concepts.