The life sciences and social sciences typically study “complex adaptive systems:” nonlinear, self-organizing, adaptive, multilevel, multicomponent systems in which dense interconnections between elements produce irreducible/emergent systems effects. Systems and their components are partially (in)separable: they can be fully understood neither solely in terms of their parts (some outcomes are emergent) nor solely in terms of the whole (the character of the parts is essential to the nature of the whole). Important implications of a complex adaptive systems perspective for IR include a new view of international systems and their structures; a distinctive understanding of social continuity and social change; new perspectives on levels, theory, and explanation; new tools for comparative analysis; renewed attention to hierarchy; and a distinctive understanding of globalization.