In this paper we use practice theory, with its focus on the interplay of structure and agency, to theorize about teacher engagement in professional learning and teacher enactment of pedagogical practices as an alternative to framing implementation research in terms of program adherence and fidelity of implementation. Practice theory allowed us to reconsider assumptions about characteristics of effective teacher professional learning, and to rethink our own notions of agency. Using data from our three-year middle school science teacher professional learning project, Language-rich Inquiry Science with English Language Learners (LISELL), we discuss how individual teachers negotiated power structures of schooling and exerted their agency in ways that were influenced by their project participation. Framing our work in terms of engagement and enactment, we theorize about how a structure-agency dialectic challenging assumptions about effective teacher professional learning can support new ways of thinking about implementation research in education.