Informal STEM education (ISE) organizations, especially museums, have used evaluation productively but unevenly. We argue that advancing evaluation in ISE requires that evaluation capacity building (ECB) broadens to include not only professional evaluators but also other professionals such as educators, exhibit developers, activity facilitators, and institutional leaders. We identify four categories of evaluation capacity: evaluation skill and knowledge, use of evaluation, organizational systems related to conducting or integrating evaluation, and values related to evaluation. We studied a field-wide effort to build evaluation capacity across a network of organizations and found it important to address individuals’ evaluation capacities as well as capacities at the organizational level. Organizational factors that support ECB included redundancy of evaluation capacities across multiple people in an organization, institutional coherence around the value of evaluation, and recognition that ECB can be led from multiple levels of an organizational hierarchy. We argue that the increasing emphasis on evaluation in the ISE field represents an exciting opportunity and that, with targeted strategies and investments, ECB holds great promise for the future of ISE and ISE evaluation.