Working from the view of schools as a system, and of school improvement as an ongoing journey (Hallinger & Heck, 2011; Jackson, 2000), this study investigated the perspectives of teacher leaders and principals from four metropolitan Catholic grade schools engaged in efforts to improve their school science programs. Built upon existing conceptualizations of school capacity, a model of school science capacity is presented and used as a framework for the development of four case studies. Findings gleaned from collecting and analyzing reports of teacher leaders' and principals' interpretations of their school systems illustrate how elements of the school science capacity framework interacted to support and at times constrain, the schools' science improvement efforts. Implications for Catholic school educators, administrators, and researchers who seek to make science a priority, and build school science capacity, are discussed.