The theoretical concept of inside activism brings fresh light on institutional change by upgrading the importance of political agency within public organizations. Inside activism captures a specific empirical phenomenon, namely, public officials being committed to the agendas of civil society networks and organizations, and acting from inside public organizations to induce policy and institutional change. Inside activism upgrades political aspects of public organizations, recognizing the importance of authority, power, and combative action. Public organizations are institutionally shaped by continuous processes of consolidation and fragmentation. This means opportunities for inside activists to act politically, preferably in secret and subversive ways, and to further strengthen the fragmented nature of public organizations. Strategically, inside activists can work for institutional change by expanding their agency through the development of collective power and networking, using combative subversive strategy, working for cumulative effects and combinative solutions as well as to bend and break constraints on their actions (the 5C model). To induce change, they further exploit institutional ambiguities like “weak spots” of institutions and discrepancies between institutional rules and practices on the ground. The neglect of inside activism within institutional theory likely means that the possibility of institutional change has been underestimated and there is thus a need for a comprehensive research agenda on inside activism, political agency, and institutional change, which in this article is termed “new political institutionalism.”