Summative evaluation plays a critical role in documenting the impacts of informal science education (ISE), potentially contributing to the ISE knowledge base and informing ongoing improvements in practice and decision-making. In response to the growing demand for capacity-building in ISE evaluation, this article presents a framework for summative evaluation based on an extensive review of literature and research-based refinements. The framework synthesizes key elements of high-quality summative evaluation into three dimensions: (a) Intervention Rationale, (b) Methodological Rigor and Appropriateness, and (c) Evaluation Uses. Judgment of the value of the intervention (e.g., program, exhibition) should draw upon all three dimensions.