The equation between the concept of society and the nation-state in modernity is known as methodological nationalism in scholarly debates. In agreement with the thesis that methodological nationalism must be rejected and transcended, this article argues that we still lack an understanding of what methodological nationalism actually is and, because of that, we remain unable to answer the substantive problem methodological nationalism poses to social theory: how to understand the history, main features and legacy of the nation-state in modernity. The first section of the article reconstructs the origins of the critique of methodological nationalism in the 1970s and differentiates between its logical and historical versions. It then critically reviews the most salient critique of methodological nationalism in contemporary social theory, that of Ulrich Beck. The final part of the article assesses the thesis of social theory’s immanent methodological nationalism by demonstrating how social theory’s equivocations towards the nation-state only mirror the nation-state’s own ambivalence within modernity. It is shown that social theory’s ambivalent attempts at conceptualizing the nation-state reflect the actual ambivalence of the position of the nation-state in modernity: its historical opacity, its sociological uncertainty and its normative ambiguity.