Ideal and non-ideal theory refer to distinct, but not necessarily mutually exclusive, ways of engaging in ethical theory, political philosophy and political theory. Ideal theory concerns the justification of some ideal normative requirements—ideal principles of social justice, for example. Non-ideal theory, by contrast, refers to the grounding of normative requirements in situations in which the ideal normative requirements are not met or are violated. Some ethicists, philosophers and theorists defend a complementary understanding of the relationship between ideal and non-ideal theory. Others, by contrast, maintain exclusive understandings of ideal and non-ideal theory that view them as theoretical endeavours which are independent from and compete with each other. This article shows the importance of the ideal/non-ideal theory distinction in the ethics of migration by focusing on the open v. closed borders debate. It illustrates that both ideal and non-ideal theoretic argumentations can be employed to either defend or reject states’ moral right to close their borders. In addition, the article explains positions that combine an ideal theoretic argumentation in favour of open borders with a non-ideal theoretic argumentation in favour of closed borders and, vice versa, positions that combine an ideal theoretic argumentation in favour of closed borders with a non-ideal theoretic argumentation in favour of open borders. In this way, the article reveals that clarifying the precise use of ideal and non-ideal theory is necessary in order to properly understand and assess the various positions in the ethics of migration.