Comparative studies of electoral institutions have largely neglected a fundamental characteristic of most of the world's electoral systems: malapportionment. This article provides a method for measuring malapportionment in different types of electoral systems, calculates levels of malapportionment in seventy-eight countries, and employs statistical analysis to explore the correlates of malapportionment in both upper and lower chambers. The analysis shows that the use of single-member districts is associated with higher levels of malapportionment in lower chambers and that federalism and country size account for variation in malapportionment in upper chambers. Furthermore, African and especially Latin American countries tend to have electoral systems that are highly malapportioned. The article concludes by proposing a broad, comparative research agenda that focuses on the origins, evolution and consequences of malapportionment.