Traditionally mathematical subject matter knowledge for teachers was defined with reference to the academic discipline of mathematics. Since the 1990s, however, a reconceptualization of what Shulman has called content knowledge for teachers has been suggested. In mathematics education, it has been conceptualized by some researchers through the notion of mathematics-for-teaching: the mathematics teachers need to know derives from the practice of teaching mathematics at the school level. The study reported upon in this article inquired into the mathematics-for-teaching used by elementary school teachers during joint lesson planning sessions. Implications for teacher education are discussed.