The first years of teaching can be demanding, as the novice works to gain familiarity with many aspects of professional work. Often, what they have been prepared to do in teacher education programs is not aligned with what is expected at them in schools. As a consequence, many teachers leave the profession or move away from the reform-minded practice emphasized in their preparation as they accept the norms of their new community of practice. Using cultural historical activity theory, this comparative case study explored two beginning science teachers' transition into the teaching profession and the roles a community of practice played in this transition. Findings suggest that these science teachers were influenced by the social and professional structures at their schools and by the level of institutional and interpersonal support they received from peer teachers and administrators. The extent to which these teachers became involved in the community of practices at their schools greatly influenced their instructional practice. Implications suggest that science teacher preparation and induction programs must go beyond simply emphasizing teaching and learning—they must also address strategies young teachers can use to access the support they need to be able to consistently enact reform-based practices. © 2009 Wiley Periodicals, Inc. Sci Ed93:996–1025, 2009