This toolkit contains a set of professional development materials whose goal is to help mathematics teachers in grades 6-10 learn to identify, describe, and foster algebraic thinking in their students. A core belief underlying the Toolkit is that good mathematics teaching begins with understanding how mathematics is learned, so these materials focus on how students think about mathematics and helping teachers to understand students' thinking through the analysis of different kinds of data such as student work and classroom observation. Instructional implications are also considered from the perspective of an understanding of how algebraic thinking develops. This material is a self-study guide and offers the following: (1) hands-on investigation, discussion, and reflection aimed at a deeper understanding of algebraic thinking; (2) language for talking and thinking about algebraic thinking; (3) structured approaches to gathering and analyzing data about students' mathematical thinking; (4) structured approaches to discussion among teachers about mathematics, student thinking, and other issues related to teachers' practice; and (5) mathematics problems that both elicit and develop algebraic thinking. (KHR)