Illegal immigration is the migration of people into a country in violation of the immigration laws of that country or the continued residence without the legal right to live in that country. Illegal immigration tends to be financially upward, from poorer to richer countries. Illegal residence in another country creates the risk of detention, deportation, and/or other sanctions.

Asylum seekers who are denied asylum may face impediment to expulsion if the home country refuses to receive the person or if new asylum evidence emerges after the decision. In some cases, these people are considered illegal aliens, and in others, they may receive a temporary residence permit, for example with reference to the principle of non-refoulement in the international Refugee Convention. The European Court of Human Rights, referring to the European Convention on Human Rights, has shown in a number of indicative judgments that there are enforcement barriers to expulsion to certain countries, for example, due to the risk of torture.