After six years of Arms Trade Treaty (ATT) implementation, and with the pace at which states are joining the treaty having slowed, the time is right to take stock of efforts to universalize the ATT. Universalization efforts to date have centered around adding as many new states parties as possible – not whether they are diverse, representative, or contributing to the development of standards and norms of responsibility, transparency, and accountability in the international arms trade. By taking stock of the status of ATT universalization; examining past, present, and future universalization efforts; and identifying challenges to universalization and strategies for overcoming them, this paper offers guidance for meaningfully expanding ATT membership without undermining the treaty’s aims.