In education policy, the irresistible force paradox—what happens when an irresistible force meets an immovable object?—resonates with many characterisations of the dynamics of education reform and the role of teachers’ unions in reform processes. According to many theories of teachers’ unions in the United States, the paradox is resolved in favour of the immovable object: strong teachers’ unions are alleged to block necessary reforms, hampering school effectiveness and efficiency. This research tests these claims about teachers’ unions, and their impact on reform outcomes—particularly, performance-related pay. Despite teachers’ unions’ supposed opposition to performance-related pay (PRP), there are a number of cases—both within and beyond U.S. borders—where PRP has been implemented. By exploring some of these ‘exceptional’ cases, this research outlines the conditions under which reform is likely to occur, and a more specific explanation of reform ‘failure’. It finds that, though education reform is often portrayed as a power struggle between reform proponents and opponents, there are multiple pathways through which reform may occur. Overpowering unions might be one route, but reform can also be secured through cooperation and compromise. These insights have significant implications for theories of teachers’ unions’ strength, preferences, and policy impact. The findings demonstrate that the insufficiency of existing theoretical accounts. Neither union preferences nor power are simple, monolithic, or predictable, and teachers’ unions do not and cannot block reform at all times and in all places. Beyond theory, these findings carry weighty implications for practitioners regarding the role of unions in public policy decision-making.