This introduction presents an overview of key concepts discussed in the subsequent chapters of this book. The book addresses a series of topics of a more general nature. Damian Courtney discusses the statistical difficulties inherent in estimates of contemporary migration and the inadequacy of the old 'residual' method of calculating migration flows. He highlights the role of the new Quarterly National Household Survey in providing new and more accurate data and discusses a number of other new data sources such as child benefit statistics, the register of electors and school enrolments. The book constitutes a modest addition to the growing scholarship in the field of Irish migration studies. It seeks to adopt a comparative world approach to Irish migration, considering the Irish migrant experience at different periods in Britain, the Americas and the British Empire as well as offering fresh perspectives on statistical, theoretical and comparative issues.