In this paper I present a case study of one teacher, Harriet, and how her beliefs and practices regarding "equity" and "social justice" in mathematics developed over several years. I use Valsiner's (Culture and the development of children's action: a theory of human development, 2nd edn, Wiley, New York, 1997) zone theory to examine the evolution of Harriet's views and practices of mathematics teaching that occurred during her participation in a long-term professional development program focused on supporting teacher leadership. Early in the program she shifted from advocating for and using direct instruction to taking a student-centered approach to mathematics. Initially, when presented with a range of perspectives on issues of equity and social justice, Harriet took up a view of equity as student-centered mathematics and actively resisted teaching mathematics for social justice. However, later in the program she began exploring teaching mathematics for social justice and came to see some value in it. I discuss mechanisms that supported her transitions and implications for the design of professional development and teacher education.