The complexity of the modern world makes it difficult to predict the effects of political actions. In his 1992 book, System Effects, Robert Jervis underscored this difficulty by pointing to various sources of complexity when people interact. For example, they may misperceive each other’s perceptions, leading their actions to backfire or create unintended side effects. In this collection, scholars of international relations, law, network analysis, political philosophy, and political science examine why questions of societal complexity have become unfashionable in some social sciences and fashionable in others. And they discuss whether complex social interactions tie our hands: if our actions are unpredictable, should we, and can we, stop acting? Among the contributors are noted legal theorist Richard Posner; Philip E. Tetlock, the world’s leading expert on the predictive shortcomings of "experts"; and Jervis himself, who contributes a retrospective look at his 1992 book and its lessons.This book was originally published as a special issue of Critical Review: A Journal of Politics and Society.