Purpose: Attending to teachers' instructional contexts is integral for understanding their learning and implementation of education reform. However, there is still much to be understood about the processes that surround teacher decision making and their navigations through change in their work contexts. Research Methods/Approach: This article utilized sensemaking theory to explore decision-making dynamics for eight middle school science teachers adopting instructional practices associated with the "Framework for K-12 Science Education" and Next Generation Science Standards. Using a multiple case study approach, I examine patterns of teachers' sensemaking and their connection to instructional decision making over a 2-year period, with particular attention to the sources of ambiguity and uncertainty that initiated sensemaking and how teachers navigated them. Data include interviews, classroom videos, instructional artifacts, and professional development workshop field notes. Findings: Teachers' institutional knowledge served as a key resource for sensemaking that supported noticing what was new about reform and managing conflicting priorities. Experiencing and resolving ambiguity supported by institutional knowledge tended to lead to greater shifts in classroom practice. However, an absence of institutional knowledge tended to result in feelings of "stuckness" that impeded change. I describe teachers' navigational strategies to resolve ambiguity, moments of stuckness, and how such experiences shaped the ways reform ideas became integrated into practice. Implications: Designing opportunities for teachers to notice discrepancies and engage in sensemaking, and leveraging teachers' local expertise and ingenuity, can support teacher learning and change to classroom practice.