This collection by an international array of historians examines agrarian radicalism in comparative context from 1500 to the present. What unifies the studies presented here is a shared interest in the ways in which agrarian people in the Atlantic world interacted with each other, transmitted and translated ideas, developed new crops or methods, or formulated critiques of the existing social, economic, and political order. All agree, to varying extents, that the Atlantic world is best conceptualized not as a rigid barrier between nations, peoples, and cultures, but rather a frontier, a permeable space with eddles and currents of ideas, cultivars, and human beings. And, as these essays indicate, radicalism can be found not only in the political realm, but also in the rate and extent of social, economic, and environmental change.