An illegal immigrant in the United States is an alien (non-citizen) who has entered the United States without government permission and in violation of United States Nationality Law, or stayed beyond the termination date of a visa, also in violation of the law.

The illegal immigrant population of the United States in 2008 was estimated by the Center for Immigration Studies to be about 11 million people, down from 12.5 million people in 2007. Other estimates range from 7 to 20 million. According to a Pew Hispanic Center report, in 2005, 56% of illegal immigrants were from Mexico; 22% were from other Latin American countries, primarily from Central America; 13% were from Asia; 6% were from Europe and Canada; and 3% were from Africa and the rest of the world.

Illegal immigrants continue to outpace the number of legal immigrants—a trend that's held steady since the 1990s. While the majority of illegal immigrants continue to concentrate in places with existing large Hispanic communities, increasingly illegal immigrants are settling throughout the rest of the country.

Immigration to the United States is a complex demographic phenomenon that has been a major source of population growth and cultural change throughout much of the history of the United States. The economic, social, and political aspects of immigration have caused controversy regarding ethnicity, economic benefits, jobs for non-immigrants, settlement patterns, impact on upward social mobility, crime, and voting behavior. In 2006 the United States accepted more legal immigrants as permanent residents than all other countries in the world combined. After ethnic quotas on immigration were removed in 1965 the number of actual (first-generation) immigrants living in the United States eventually quadrupled, from 9.6 million in 1970 to about 38 million in 2007. Over one million persons were naturalized as U.S. citizens in 2008. The leading countries of origin of immigrants to the United States were Mexico, India, the Philippines, and China. Nearly 14 million immigrants entered the United States from 2000 to 2010.

The United States of America (commonly abbreviated to the United States, the U.S., the USA, America, and the States) is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to the east and Russia to the west, across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories in the Pacific and Caribbean.

At 3.79 million square miles (9.83 million km²) and with over 312 million people, the United States is the third or fourth largest country by total area, and the third largest by both land area and population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The U.S. economy is the world's largest national economy, with an estimated 2011 GDP of $15.1 trillion (22% of nominal global GDP and over 19% of global GDP at purchasing-power parity). Per capita income is the world's sixth-highest.

Illegal immigration is the migration into a country/state in violation of the immigration laws and sovereignty of that country/state. Illegal immigration raises many political, economic and social issues and has become a source of major controversy in developed countries and the more successful developing countries.

In 2010, the International Organization for Migration (IOM) estimated that 25.5 to 32.1 million people (or 10%–15%) of the world's total 214 million international immigrants are illegal immigrants, though the IOM notes that it is difficult to make accurate estimates.