As the "Short Twentieth Century" came to an end, more and more democracies seriously considered the possibility –often for the first time in their history- of changing their national electoral system. Since then, the total number of electoral reforms enacted in countries that select their rulers through free and fair elections has sharply increased: in the last two decades over 33% of the world's democratic states modified the formula employed for choosing the members of their national legislatures, and a similar percentage adjusted other elements of the rules of the game such as the district magnitude, the legal threshold, the assembly size or the ballot structure. Unfortunately, the academic examination of the causes and consequences of these episodes of institutional change has lagged well behind these empirical developments with single case studies and small n studies still the norm. In light of this gap, research on the determinants and the outcomes of electoral reform processes is increasingly needed. The aim of this study is to transcend the analysis of a small number of cases, and instead to comparatively examine the universe of electoral system changes that have occurred in 60 contemporary democracies between 1945 and 2010. The thesis has three main findings. First, the levels of party system fragmentation and citizens' satisfaction with democracy have strong potential to explain electoral system changes in contemporary democracies. Contrary to what is usually implied by the literature on electoral reform, parties are seen to have strong tendencies to pass restrictive rather than permissive electoral system changes in circumstances where the electoral system might be considered to be already overly-permissive resulting in excessive numbers of parties. Moreover, electoral reforms in the intraparty dimension usually take place when large numbers of voters are currently dissatisfied with the way democracy works in their country. The second main finding is that electoral reforms can reshape the morphology of established party systems through two distinct mechanisms of electoral engineering. The first mechanism takes place at the interparty level, with permissive reforms reducing the difference between the percentage of votes received and the percentage of seats obtained by a party, and restrictive reforms enlarging this gap. The second mechanism operates at the intraparty level, where candidate-centred reforms decrease the level of party system nationalization while partycentred reforms leave party system nationalization unchanged. Finally, the third main finding of the thesis is that parties' positions regarding the possible modification of the rules of the game have an electoral effect. Parties that advocate a permissive reform in countries with low party system fragmentation are more likely to electorally benefit. By contrast, support for such reforms when the number of parties is large is more likely to result in electoral loses.