An illegal immigrant is somebody who illegally enters a country and takes up residence there. The status and rights of illegal immigrants as well as the morality of both illegal immigration and the hiring of illegal immigrants are controversial subjects. Those who are opposed to strict policies on immigration, and who support the rights of immigrants and migrant workers, generally use the term "undocumented worker" or "undocumented immigrant" rather than "illegal immigrant".

It is estimated that over 50% of US crop workers are illegal immigrants.

People who are persecuted in their original country and then enter a country seeking safety are often said to be refugees or "seeking asylum". Some of those who are opposed to strict policies on immigration characterize all illegal immigrants as legitimate refugees, not limiting the term to political refugees.

Illegal immigration is largely driven by economic forces, especially unemployment and globalization.

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