Vertical integration is an important concept for political parties. In multi-level or federal contexts, it is said to affect party strength, national integration and federal stability. Despite this, difficulties with the conceptualization and operationalization of vertical integration and a lack of cross-national data impede research. This article clarifies the concept of vertical integration, distinguishing it from related concepts of strength, centralization and autonomy and distinguishing the indicators of integration from the effects of integration. It introduces the measures of vertical integration and autonomy used in the Party Organization in Multi-Level Systems (POMLS) dataset comprising data from survey responses from 204 state-level parties in eight countries. The data confirm the theoretical distinctions among forms of vertical integration and between vertical integration and autonomy and show that not all forms of vertical integration are mutually reinforcing.