As an accompaniment to the translation into English of Louis Althusser's 'Letter to the Central Committee of the PCF, March 18th, 1966', this note provides the historical and theoretical context necessary to understand Althusser's 'anti-humanist' interventions into French Communist Party policy decisions during the mid-1960s. Because nowhere else in Althusser's published writings do we see as clearly the political stakes involved in his philosophical project, nor the way in which this project evolved from a 'theoreticist' pursuit into a more practical one, the note also argues that the letter is of importance to Althusser scholars, to historians of Marxist thought, and to those interested in the relevance of Althusser's work to contemporary Marxist philosophy.