This thesis examines why party systems of some developing countries become deeper and more quickly stabilised than others. Drawing on the scholarship of party system institutionalisation in third wave democracies, the thesis argues that the field can be strengthened by looking to three factors that the current literature has taken for granted: the role of cleavages, the function of personalistic politics, and the requirement of legitimacy to assess party systems. This thesis addresses these issues and in so doing provides a novel view of how, when and why party systems in newer democracies from Latin America, Europe and Asia consolidate over time.The research considers three case studies from three regions of the world, following the most similar approach method of comparison. One country per world region has been chosen for study in detail: Chile (Latin America), Estonia (Central and Eastern Europe) and South Korea (East and Southeast Asia). They all have party systems which have become more stable over time, but they exhibit different trajectories and speeds of consolidation. The thesis uses a variety of methods. In order to infer the causes of different processes of institutionalisation from party systems' own participants, more than 120 elite interviews were conducted in the three countries over 13 months. To evaluate the overall legitimacy of the stabilisation process, this works presents the results of almost 500 face-to-face interviews with randomly selected individuals from the population. Quantitative analyses based on secondary public opinion surveys are used to test implications and observations, and offer potential generalisations.The findings suggests: 1) Where the ideological cleavage (left-right) is a strong determinant of party support the party system is more stable, and the stronger the ideological cleavage becomes over time, the more consolidated the party system is. Here, an ideological trauma can be at the core of the limitations of the left-right scope development. 2) Party systems with personalistic leaders can consolidate, contrary to the received wisdom, if charismatic figures build their parties around programmatic lines. And 3) legitimacy should not be regarded as a dimension for the Theory of Party System Institutionalisation, because it does not contribute in any way, positive or negative, to the stability of party systems. The thesis concludes that theories of party system institutionalisation should be reconsidered with respect to cleavages, personalism and legitimacy. In so doing, the growing literature on party system institutionalisation can benefit from a more comprehensive understanding of the complexities of party systems in new democracies from different regions of the world.