Migration is part of the human condition and is a fundamental fact with which political theory must contend. Where economic or other gradients are sufﬁciently strong, they create what is essentially an irresistible force for migration. In such cases democratic states tend to ﬁnd it impossible to stop illegal immigration, if legal immigration is limited. Yet contemporary political theory debates immigration in terms of ideas which prescind from these realities, and which moreover are not always interpreted correctly in their implications for immigration. Three such ‘immovable ideas’ – sovereignty, democracy, and nationality – are explored in this chapter. I do not pretend here to offer a comprehensive approach to immigration in normative terms, but rather survey grounds for scepticism about the way these three values are commonly invoked in relation to immigration. Before turning to these normative considerations, the remainder of this introduction elaborates on the sense in which migration may count as an ‘irresistible force’, one which both precedes and in crucial respects supersedes modern states.