A vast literature examines how geopolitical alignments influence militarized conflict, trade, investment, aid, and other outcomes in international relations. Yet, considerable ambiguity remains regarding how such alignments are conceptualized and how existing measures correspond to distinct theoretical understandings. This article clarifies the conceptual foundations of geopolitical alignment along two key dimensions. First, some measures capture ideological contestation over the global order, whereas others reflect political agreement or rivalry over specific issues, such as territorial disputes. Second, some measures conceptualize alignment as agreement (e.g., similarity in UN voting), while others treat it as a relationship (e.g., alliances or arms transfers). These distinctions correspond to alternative understandings of alignment that carry important implications for empirical research. For instance, Saudi Arabia appears to be aligned with China when alignment is defined ideologically, but with the United States when defined by security ties. The article provides guidance on selecting appropriate measures for different conceptualizations and introduces a new indicator based on the ideological profiles of a state’s arms suppliers.