This chapter argues that general principles outlining how to design professional development, although potentially helpful, are not sufficient for meeting the needs of teachers. It highlights the complexity of professional development and the necessity of understanding the details around engaging teachers in learning opportunities that lead to generative growth. The chapter provides the opportunity to think carefully about the role of the mathematical content in the design and implementation of professional development. It discusses how the algebraic thinking professional development work would differ from our earlier whole number professional development. The chapter details some of the critical issues and how chapter addresses them. It characterizes our whole number professional development and contrasts it with our algebraic thinking work. The chapter articulates in what particular ways content may make a difference in the design of learning opportunities for elementary school teachers. It recognizes that professional development for teachers requires constant attention to and explicit articulation of the mathematical content.