In this important book, Thomas R. Flynn reinterprets and evaluates Sartre's social and political philosophy, arguing that the existential ethics of Sartre's early phase is consistent with the Marxist-inspired views of his later writings. Displaying his mastery of Sartre's entire corpus, Flynn reconstructs Sartre's social ontology with its sensitive balance of the existentialist's respect for moral responsibility and the Marxist's sense of social causation. Flynn focuses on the issue of collective responsibility as a particularly apt test-case for assessing any proposed union of existentialist and Marxist perspectives. The study begins with an examination of the uses of "responsibility" in Being and Nothingness and in several postwar essays. Flynn then concentrates on the Critique of Dialectical Reason, offering a thorough analysis of the remarkable social theory Sartre constructs there. A masterful contribution to Sartre scholarship, Sartre and Marxist Existentialism will be of great interest to social and political philosophers involved in the debate over collective responsibility.