Throughout time, immigrants have shaped the history of the United States of America. As a country founded by immigrants hundreds of years ago, many individuals still flock to the U.S. today seeking political freedom and the opportunity to achieve their goals. America remains a land of opportunities, and because many foreigners from all over the world come to live in the U.S., cultural diversity is cited as one of the country's strengths. Immigration and immigration enforcement policies remain controversial and divisive topics. Immigration also has a unique history in the United States. The United States did not always have to worry about illegal immigration and immigration enforcement. Instead, in the past, the country encouraged immigration when they the country was full of vast, open spaces and primitive transportation and communications. As transportation became more prominent and transportation costs fell, foreigners found it more attractive to migrate to the United States.

After the Civil War ended in the United States, states were faced with issues pertaining to immigration and thus, began introducing immigration policies. As a result, in 1875, the U.S. Supreme Court designated the regulation of immigration policies to the federal government and not individual states. The first big wave of immigration to the United States began in the 1880s, and the government reacted by creating the Immigration Service in 1891. When World War I began the number of immigrants coming to the United States dropped significantly, especially regarding immigrants from Europe. After the war ended, the number of immigrants started to increase again. To deal with the high number of immigrants entering the country, the U.S. Congress introduced new immigration policies. The government introduced the National Origins Quota Act in 1921. The quota limited the number of legal immigrants to 3% of their current ethnic makeup in the United States. This immigration quota system was altered three years later and the percentage was lowered to 2%. In addition, Congress established the U.S. Border Patrol as part of the Immigration Service.

In addition to immigration rates dropping during World War 1, immigration levels also dropped during the Great Depression. In fact, the country experienced a time period of zero growth from immigration, because no one desired to come to a country that was experiencing a prolonged period of harsh economic conditions and uncertainty. While immigration increased after World War II, the rates remained relatively low. This continued for 20 years. In 1952, all the previous immigration policies and laws introduced by Congress were combined into the Immigration and Nationality Act. Due to the large number of seasonal agricultural workers from Mexico, the U.S. and Mexican governments introduced the Bracero Program. In the following years, more immigration legislation followed. In 1965, Congress transformed the National Origins Act and gave immigration preference to families so that they could be united in the U.S. In addition, proponents of the legislation believed the system would bring in a higher number of skilled immigrants to the U.S. In the past, the majority of immigration visa applicants were of European descent. Today that is no longer true and the majority of immigration applications are received from Asia and Latin America. The availability of immigration visas annually continues to be limited by the U.S. government. In the past, the U.S. did not have a specific policy regarding refugees, but this changed when the Refugee Act of 1980 was introduced.

The number of illegal immigrants attempting to come into the United States has continuously increased. In addition, the number of legal immigrants admitted into the country has reached new highs. It is estimated that legal immigration in the 1990s surpassed the levels of the last previous peak of legal immigration from 1901 to 1910. During that time period nearly 9 million legal immigrants were allowed into the United States. From the period of 1968 to 1993, it is estimated that 16.7 million immigrants entered the country legally. Of these 16.7 million legal immigrants, nearly 85% were from developing countries. This percentage is composed of nearly 50% legal immigrants that came from the Caribbean and Latin America and about a third that came from Asia. Throughout the 1980s and 1990s some illegal immigrants in the U.S. have benefited from immigration policies that have granted amnesty, created a system for refugees, and have raised the quotas for the number of legal immigrants allowed. The number of legal immigrants allowed is anywhere from 700,000 to 900,000 on an annual basis.

While immigration rates to the United States have consistently risen since the Great Depression, a recent trend has emerged. Due to the dim economic climate in the United States, officials say that an increasing number of immigrants are returning home to Latin America. Immigrants, both legal and illegal, reportedly have no choice but to return to their native countries due to the limited number of jobs in employment sectors that predominantly attract recent immigrants such as the restaurant, landscaping, and construction industries. Immigration officials in Mexico City note that while immigration tends to be cyclical, with many immigrants returning home for the December and January holidays, they are already seeing an increase in returning immigrants. In fact, it is estimated that 30,000 additional immigrants will leave the United States due to the decrease in employment opportunities. Officials remark that Mexican immigrants who have lived in the U.S over two years rarely return home, as they generally have established familial connections in the U.S. and employment prospects are even worse back home. In addition to the increase in immigrants leaving the United States, immigration to the United States has decreased in recent years. According to the U.S. Border Patrol, apprehensions of illegal immigrants were down 18 percent from 2006 and almost 40 percent from 2005.

Overall, however, immigration to the United States remains popular, as there are an estimated 12 million undocumented immigrants residing in the country. The department that deals with immigration services and benefits is called the USCIS, or U.S. Citizenship and Immigration Services. The USCIS was formerly known as the INS or Immigration and Naturalization Service. In March 1, 2003 the USCIS became a part of the Department of Homeland Security. However, the immigration enforcement responsibilities fall under the Bureau of U.S. Immigration and Customs Enforcement, or ICE.