The largest period of migration to the United States occurred between 1870 and 1920. During that period, more Poles migrated to the United States than any other national group except Italians. Additional large-scale Polish migration occurred in the wake of World War II and during the period of Solidarity's rise to prominence. According to current estimates, at least nine million Americans trace their roots to Poland. This encyclopedic reference work has three types of entries: thematic essays, topical entries, and biographical entries. The essays synthesize existing work to provide interpretations of, and insight into, important aspects of the Polish American experience. The topical entries identify specific places, events or organizations such as the Polish National Alliance, Polish American Saturday Schools, and the Latimer Massacre, among others. The biographical entries identify Polish Americans who have made significant contributions at the regional or national level either to the history and culture of the United States, or to the development of American Polonia.