The contestation of international organizations (IOs) over past decades has inspired a rich research program. This paper takes stock of progress by reviewing key findings in existing scholarship and identifying critical areas for future study. Examining patterns, drivers, and consequences of IO contestation, we advance three key arguments. First, contestation occurs through state and societal actors calling for reform, replacement, or rejection of IOs. Yet, contrary to common expectations, various measurements of contestation do not show a general rise; instead, it shows extensive fluctuation and variation. Second, contestation of IOs originates from drivers at both international and national levels, ranging from global power shifts and international authority to democratic decline and populist entrepreneurship. These drivers interact in producing complex dynamics of contestation across levels. Third, contestation has both short-term and long-term consequences for IOs. Nevertheless, overall, IOs have remained surprisingly resilient in the face of state and societal contestation. Going forward, this research program needs to further explore the origins of the expectations-findings gap, the nature of interactions among drivers, and the broader effects of contestation on global order.