Recent advances in the availability of parliamentary data across countries, such as speeches or roll-call votes, have led legislative scholars to increase their focus on the attitudes and behavior of individual Members of Parliament (MPs). This allows us to have a better understanding of topics such as (re)selection, rebellion or compliance with the party line, leadership contests, and substantive representation of women. However, due to data restrictions, the distribution of power within parliamentary parties has often been restricted to a leadership-backbench dichotomy. We propose to go significantly beyond this definition. Parliamentary power is defined by both role and policy area: an MP who chairs the Budget and Taxes committee is certainly more prestigious within parliament and party than a backbencher who is member of the Sports committee. This has important implications for MPs' behavior in areas such as party loyalty, representation, and careers. By collecting original data on committee membership for 19 democracies between 1987 and 2020, and using a hierarchy across policy areas, we propose an index that captures the aggregate level of parliamentary power for each member of parliament in relation to their party: the Parliamentary Power Index. We show that this measure is cross-nationally comparable and demonstrate that the PPI is a valid and reliable measure, which correlates with other indicators of prestige. Moreover, by making available the original dataset with individual MPs' committee assignments across nineteen countries for more than three decades, we also provide researchers with an important tool to study legislators' behavior.