Grounded in a belief in the transformative possibilities of social studies teacher education, I present an argument for considering the development of purpose as both content and pedagogy of social studies teacher education. As part of developing a coherent pedagogy of rationale-based social studies teacher education, I begin by exploring the influence of competing conceptions regarding the purpose of social studies teaching and learning on the work of social studies teacher educators. Next, I consider three distinct paths social studies teacher educators have taken to prepare teacher candidates for content and pedagogical decision-making as first-year teachers. While each has potential, I contend that they each miss an important first step--considering why teacher candidates want to teach social studies. Drawing on a growing body of research on rationale development, and my work as a teacher educator, I propose forging a new path. Situated in rationale-development as a core theme of social studies teacher education, and built on a conception of purpose as content and pedagogy, this path is designed to bridge the gap between rationale development and the subsequent rationale-based practices of beginning teachers.