Adolf Behne (1885 – 1948) was a German avant-garde art critic who, during the 1910s and 1920s, remained an “überzeugter Sozialist.” He was also an important theorist for the neues Bauen architectural movement in Germany. Several architects and critics of the avant-garde declared themselves socialists, especially during the Weimar Republic; Behne was certainly not the only one. Yet, if we want to understand in greater detail the ways that German modernists conceptualized the nature of architecture’s socio-political agency, Behne’s texts prove to be among the most important. His writings reveal the grandiosity of the political ambitions of the neues Bauen movement and, unlike most other figures who advocated for the new architecture, Behne was focused most of all on explaining to readers that and how the new architecture had socio-political agency, or a means of influencing politics. This dissertation examines Adolf Behne’s writings on architecture between 1912 and 1933 from a Humanist Marxist perspective. I show that his work was primarily about the question of architectural agency, that his theory was guided by a humanistic idea of socialism, and that he sought to outline what I call a ‘constitutive’ theory of agency. To explain this, I analyze his definitions of architecture as ‘bauen’ and ‘Lebensgestaltung.’ This study also demonstrates that two conceptual models of architectural agency dominate architectural history today: ‘reflectionism,’ which holds that architecture can reflect politics, and ‘productivism,’ which maintains that architecture can produce a new society. Behne, influenced by Neo-Kantianism and Life-Philosophy, instead believed that architecture had agency insofar as it constituted human reality. I conclude that, even if parts of his theory may no longer be appealing, Behne’s writings have problematized the concept of architectural agency and exposed the limited ways in which historians have conceptualized architecture’s relationship to society and politics, past and present.