Science museums and science centers exist (in large part) to bring science to the public. But what public do they serve? The challenge of equity is embodied by the gulf that separates a museum's actual public and the more diverse publics that comprise our society. Yet despite growing scholarly interest in museums and science centers, few researchers have explored how these organizations seek to bridge that gulf. Adopting an institutional theory perspective, we argue that equity is a field-wide challenge in informal science education--a challenge that different organizations define and respond to in different ways. We draw on interviews with leaders from fifteen museums and science centers around the United States to examine how equity work reflects emerging norms of practice as well as local influences. Finally, we describe two institutional logics, client logic and cooperative logic, that contain different ideas about the relationship between an organization and its publics. These logics affect both the definition and the practice of equity work. The tension between them evokes a broader tension between dissemination and participation in public engagement with science.