An estimated 11.5 – 12 million foreign nationals are living illegally in the US, according to the Pew Hispanic Center.
These are mainly people who have entered the US without the necessary documents or who have overstayed temporary visas.
The pace of unauthorized arrivals has accelerated over the last two decades. On average, nearly five times as many illegal immigrants enter the country each year now as did in the 1980s. About 40% of illegal immigrants have been in the US five years or less.
More than three quarters of illegal immigrants in the US were born in Latin America, with a large proportion of these coming from Mexico.
Most enter the US by slipping across the 2,000-mile (3,200 km) southern border – in 2005 more than a million people were arrested trying.
Although the typical illegal immigrant is often perceived as young, male and traveling alone, unauthorized migrants range from whole families, including children, to lone women.
Two-thirds of all illegal immigrants in the US are concentrated in only eight states. The large border states California, Texas and Florida host the biggest populations.
Overall, illegal immigrants account for 3.6% of the total US population.
The states where illegal immigrants make up the largest percentage of the state population are Arizona (8%) and Nevada (7.5%), followed by California (6.9%), Texas (6.3%) and Florida (5.2%), according to Pew Hispanic Center figures.
About two thirds of illegal immigrants in the US work, making up 4.9% of the total US workforce. They tend to be concentrated in occupations which require little formal education and demand no official licence.
Sectors such as agriculture, food preparation, transport and construction attract large numbers of illegal immigrants. Those areas include particular trades, such as insulation installation, roofing, meat and fish processing and garment pressing, in which immigrants account for more than a quarter of the workforce.