This chapter explains party system aggregation as the extent to which the political parties in the system represent broad political interests. The chapter uses NPPlog to operationalize party system aggregation in generating hypotheses H3.1 through H3.6. Unlike party system competitiveness and stability, party system aggregation exerts no significant impact on Rule of Law (RL). As with RL, party system aggregation has no significant effect on Government Effectiveness and Control of Corruption. The concept of interest aggregation is normally associated with individual political parties, but it also has application to party systems. Scholars have historically classified party systems by the number of parties that regularly contest elections and have recently created more elaborate classifications. The chapter reviews five operational measures: strength of the largest party, number of parliamentary parties, Rae's Fractionalization Index, the Laakso-Taagepera formula for effective number of parliamentary parties, and Mayer's Aggregation Index.