This research note discusses limitations of principal-agent (PA) analysis in explaining gradual change in international organisations (IOs). It suggests that historical institutionalism (HI) can fill important gaps left by the PA approach and identifies scope conditions for both approaches. For this purpose, a distinction is made between two sources of state power that PA usually treats as synergistic – namely the formal control of IO decisions and material power resources. While PA analysis is best applicable where reform coalitions of like-minded member states control both formal and material resources, in many contexts there exist frictions between material and formal power in IOs. In these constellations recent HI-inspired works on gradual modes of change such as ‘layering’ and ‘drift’ are of particular relevance. This research avenue is illustrated with empirical examples from a variety of international organisations.