This article develops a theory of interdisciplinarity and examines relations between historians and sociologists in Germany and France over the course of the twentieth century, focusing in on several key moments of interdisciplinary activity. Interdisciplinary engagements are motivated by scholarly problems, field-specific interests and battles, and pressures and inducements coming from states, businesses, and scientific institutions. Analysis of the most productive moments of cross-disciplinary interaction suggests that they occur when disciplines are equal in power and when scholars are motivated by scholarly problems and disciplinary conflicts to move beyond their disciplines. More generative forms of interdisciplinarity are dialogic and processual, characterized by a fusion of perspectives; less productive forms are externally induced, involve asymmetrical partners, and are organized around division of disciplinary labor rather than an interpenetration of perspectives. The most productive interdisciplinary conjunctures result from serendipitous resonances and contingent synchronicities between subfields of semi-autonomous disciplines. It is thus impossible to produce the most fruitful forms of interdisciplinarity deliberately. The article examines three cases of symmetrical, processual interdisciplinarity involving sociology and history. Two of these cases were located in the French academic field, first between the wars, and then again after 1980. The other case of dialogic collaboration between historians and sociologists begins in Nazi Germany and continues after 1945 into the 1960s, leading to the formation of West German Historische Sozialwissenschaft. Examples of unbalanced interdisciplinarity include German “History-Sociology” during the Weimar Republic, in which sociologists’ opening to history was not reciprocated by professional historians and Historische Sozialwissenschaft after 1970.