The chapter introduces the phenomenon of external voting, defined as exercising voting rights by citizens who are temporarily or permanently residing outside their country of origin. We delimit this form of electoral participation from similar forms of transnational politics and discuss the history and diffusion of expatriate enfranchisement, now covering a majority of countries in the world. We then go through the different features of external voting—where is it present, what sorts of elections are covered, what are the modes of organizing elections, how are ballots counted in different political systems, and what the criteria of eligibility might be. Finally, we review the extant literature on external voting, originating in comparative politics and in migration studies, and highlight some gaps that this book seeks to fill.