Professional development holds significant potential in promoting science education, but that potential is undermined if instructional changes are not sustained. For professional development, sustainability refers to the continuation of outcomes over an extended period of time after the program ends and is an issue across international contexts. This longitudinal research, part of a larger research project funded by NSF, investigated sustainability of early elementary science instruction after teacher participation in a 3-year professional development program and the factors that influenced teachers' decisions about instructional time and strategies. The research used a case-study approach to obtain an in-depth understanding of why instructional shifts occurred over a period of 7-years. The primary data sources were teacher surveys, self-efficacy assessments, and interviews. The findings highlight how school level factors changed over time and affected teachers' science instruction in both positive and negative ways. The research illustrates the connection between contextual constraints and science instruction and holds implications for sustaining meaningful instructional changes after professional development ends.