Currently, there are millions of Mexican nationals in the United States illegally,  and also millions of other illegal foreign nationals as well. Some have cited a population in excess of 12 million foreigners without legal status in the United States (although this figure is debatable).
In the United States and most other countries, it is illegal and punishable by law to hire a worker without legal work authorization. In the United States, it is often felt that immigration laws are either loosely or infrequently enforced. The same feeling has often been encountered in Europe, resulting in European countries tightening their immigration laws and increasing enforcement. Countries and territories such as Singapore, Australia and Hong Kong are very strict about the enforcement of immigration laws. Illegal immigrants caught working in these places can expect deportation, perhaps a prison term, and their employers may be fined heavily.
In other English-speaking countries, illegal aliens are often referred to as "illegal immigrants".