Over the past two decades, global health diplomacy, foreign policy for health, and global health policy have changed substantially. Diplomacy is a constitutive part of the system of global health governance. COVID-19 hit the world when multilateral cooperation was subject to major challenges, and global health has since become integral to geopolitics. The importance of global health diplomacy, especially at WHO, in keeping countries jointly committed to improving health for everyone, has once again been shown. Through a systematic review, this Series paper explores how international relations concepts and theories have been applied to better understand the role of power in shaping positions, negotiations, and outcomes in global health diplomacy. We apply an international relations perspective to reflect on the effect that those concepts and theories have had on global health diplomacy over the past two decades. This Series paper argues that a more central role of international relations concepts and theories in analysing global health diplomacy would help develop a more nuanced understanding of global health policy making. However, the world has changed to an extent that was not envisioned in academic discourse. This shift calls for new international relations concepts and theories to inform global health diplomacy.