Whereas science teachers often express initial enthusiasm about the student-centered, reform-oriented instructional strategies they encounter in professional development (PD), they rarely adopt them in practice. One possible obstacle is a mismatch between these strategies and teachers' beliefs about what is possible and desirable in their particular teaching contexts. In the current study, we investigated changes in teachers' beliefs about practices within a PD context, and consider beliefs about practice as one component within teachers' overall professional role identity. Using the Dynamic Systems Model of Role Identity (DSMRI) as our theoretical framework, we analyzed pre-PD, mid-PD, and post-PD interviews, daily written reflections, and responses to exploration and commitment prompts of four high school science teacher participants in an 8-day, reform-oriented summer institute. Teachers' experiences, changes in the content, structure and process of various components of their professional role identities, and their eventual endorsement or resistance to new practices, were framed by the dynamic interplay between their current science teacher role identity and other role identities. When discussing possibilities for action in subsequent teaching practices, alignment, and tensions within and between these role identities promoted or impeded changes in behavioral intentions. The findings highlight the importance of considering prior and current role identities when designing and evaluating PD contexts, and the complex and contextual nature of identity change among teachers.