This study investigates the identity development of teacher leaders in an, urban school district who participated in a Noyce Master Teaching Fellow, program. We identify tensions that arose from involvement in this, external community of practice (CoP) and changes in teacher leader, meanings and practices in their school CoPs. Qualitative key findings, indicate that as boundary crossers, (1) teacher leader activities surfaced, tensions between CoPs, promoting boundary competence, and (2), participation in an external CoP reshaped their identities, especially mentors. Implications include recognizing that external CoPs can support, learning mechanisms for addressing educational issues, for example, teacher retention.