This project examines the diverse attempts by scientific practitioners to chronicle, explain, and represent the transition between childhood and adulthood over the course of Germany’s long nineteenth century. These approaches paid particular attention to understanding the physiological and psychological components of the will—that facet of mind that had long been considered the essential index of moral freedom. Locating and measuring the will also implicated research in other areas related to cognition and human behavior, such as choice, attention, consciousness, habit, reflex, and language. Amidst the array of developmental theories—evolutionary, historical, phenomenological, and ontogenetic—that proliferated in the nineteenth century, tracing the formation of the human being through infancy and childhood contributed to the promises of education and psychology to be able to intervene in development and mold the human for the modern state. Focusing on the implications of child development studies for the administration of public education in Germany as well as debates between prioritizing practical training versus the cultivation of moral character, this study investigates the politics of pedagogy in the period of Germany’s industrial modernity. By examining scholarly microcosms of psychiatrists, pedagogues, philosophers, and psychologists, this project considers the institutional and cultural contexts, the instruments and technological apparatuses, and the various textual genres (from meticulous observations of infant life to classifications of mental disorders in children) that shaped the conceptual and physical distinctions between childhood, adulthood, and, by the end of the nineteenth century, adolescence.