This article examines how the principles of complex systems can illuminate recurring mechanisms of change in theories of international relations. It applies the logic of complex systems to two specific puzzles in international politics – the problem of theorizing change in structural realism, and the dynamics of cross-border democratic diffusion. In the first case, by shifting the analysis of anarchy's consequences from state behavior to state attributes, complex systems can illustrate the sources of domestic and international transformations embedded in structural theories. This approach offers a way to think about democratization as a global process of interstate competition and socialization driven by the pressures of anarchy. In the second case, the principles of co-adaptation in complex systems can help reframe diffusion not as the unilinear spread of democracy but as the interplay of self-reinforcing and self-dampening dynamics, whose interaction shapes both actor expectations and democratic outcomes. In both cases, complex systems serve a limited but useful role; although not conducive to theory creation, the approach provides a useful analytical prism for examining patterns of change and continuity in global processes, and highlights concrete ways of improving models of transformations in international politics.