The Introduction starts with some examples of pairs of old and new democracies, some larger, some smaller, some in the East, some in the South, and illustrates how party system development can differ despite very similar background conditions. We then proceed to identify four main factors that differ within each pair, as possible explanations of the way party politics in general, and partisan interaction in particular, evolved in different countries. After stating the aims of the book and how it builds on previous studies, we summarize its main conceptual, operational, and methodological innovations, putting specific emphasis on the importance of history and a combination of large- and small-N studies. Finally, we explain the logic and structure of the book with a brief summary of the content of each chapter.