The core of Ostrogorski's analysis is a detailed history of the rise of and changes within the party system in Britain and the United States, the first nations to introduce mass suffrage. While the emphasis of is on the similar trends in the political parties of both countries, Ostrogorski also showed concern with the sources of differences between them. Seeking to explain these variations, he suggested a number of fundamental hypotheses about these two societies that con-tinue to be of relevance today. Lipset's substantial introduction places Os-trogorski's work within its historical context and assesses Ostrogorski's im-pact and influence on both his contemporaries and on later political scien-tists.