This dissertation aims to problematize the historical concept of “social engineering.” In historiography, social engineering is usually understood as the application of scientific theory to political and social practice. As such, it is thought to have characterized much of early 20th century expansion of public interest and state responsibility into previously non-politicized areas of private life, especially when deceptive and/or technological in nature. It has also been seen as an expression of mechanistic “modernity” and “technocracy.” Through a comparative and conceptual histoire croisée of social engineering this dissertation studies how this concept was “spoken” in Sweden and the USA circa 1890-1950. The comparison shows that social engineering rhetoric emphasized the role of human agency and voluntarism in social change, rather than social laws or mechanistic determinism. As such, it highlighted the “constructed” character of the social and opened up the reach of the political. While it did indeed make an analytical separation between science and politics (in the interest of “efficiency” and “objectivity” of science) it also sought to bridge this very gap functionally (in the interest of the “justice” and “representativity” of politics). Rather than a technocratic attempt at moving against, above, or beyond politics social engineering rhetoric sought an intermediary role between science and politics as a kind of “intrapolitics.” Such a modern code of conduct, a “social diplomacy” of sorts, strove to bring opposed social interests into controlled intercommunication with one and another instead of promising a Utopian end to all conflict. Thus, social engineering ran against both the laissez-faire liberal ideal of a harmonious balance between various interests as well as the socio-biological and historical materialist doctrines of an apocalyptic conflict between classes and/or races. When these ideologies were cornered as a result of World War II and the Cold War, social engineering rhetoric also lost much of its raison d’être and faded away from public discourse.