This paper argues that the reading of Althusser which finds a pronounced continuity in his conception of the relations among science, philosophy, and politics is the correct one, this essay will begin with an examination of Althusser’s “scientism.” The meaning of this term (one that differs slightly from contemporary usages) will be specified before showing how and in what way Althusser’s political philosophy between 1960 and 1980 can be described as “scientistic.” The next section details the important political role Althusser assigned to the sciences and particularly to the science of historical materialism during this period. This accomplished, the arguments of interpreters who emphasize the apparent difference in Althusser’s attitude towards science before and after 1980 will be considered. Here, possible reasons for such a reading will be rehearsed. Next, with the support of recently published and archival documents, this essay will engage in a close and comparative reading of Althusser’s texts from the 1970s and 1980s that have as their subject the relations among philosophy, science, and politics. This survey will show the continuity in Althusser’s position vis-à-vis the sciences: namely, that if we want good (i.e. desired) socio-politico-economic changes to result from our political actions, then it is necessary to engage in social scientific research or, at the very least, to consult such research and to use this knowledge in our political decision making. All this serves to support the conclusion that Althusser’s “new” political philosophy from the 1980s is not really so new. On the contrary, his writings on the materialism of the encounter and aleatory materialism represent prolongations and elaborations of positions and ideas already developed in the 1960s and 1970s and that include a mostly consistent understanding of the relations between scientific knowledge and political action. This is true even if the rhetorical and philosophical style in which these ideas are put forth in the 1980s differs from the ways in which these ideas were introduced during the prior two decades.