The earliest migration to the continent took place about 20,000 years ago, but it is Christopher Columbus who first brought to the attention of Europeans the new world. Driven by the desire of seeking freedom and wealth, tens of thousands of immigrants moved from Europe to the rich land, establishing the base of a new civilization. Most immigrants landed at one of the five major American ports: New York, Boston, Philadelphia, Baltimore and New Orleans. In 1776’ Thomas Paine, an English immigrant, wrote a famous line: *'Europe, not England, is the parent country to America." Besides these European immigrants, there was a large body of African immigrants, who were forced to move to the continent as slaves through slave trade. Between 1619 and 1808, about 500,000 Africans were brou^it to the colonies as slaves, and by the eve of the Civil War, there were more than one million slaves in the U.S.

There occurred three times of immigration waves since 1620. The first happened between 1820 and 1860，when the number of immigrants amounted to 5 million. The immigrant population consisted of 2 million Irish, 1.7million Germans，thousands of African slaves and some Asians, mainly Chinese. The second wave came from 1861 to 1880, when skilled laborers were in great demand with the rapid development of American industrialization. About 5 million people from European countries came to the new continent. The third wave occurrcd between 1881 and 1920，and the number of immigrants rose to 23，500,000, most of whom came from such South-eastern European countries as Italy and Russia. As a result, the American population, for the first time in history，adds up to more than 100 million.

According to the first official census in 1790, almost half of the population about 2,000,000 people had Engish origin and the rest came from different origins, such as Scots-Irish, German, Dutch, French, Swedish, Welsh and Finnish. These white Europeans were mostly Protestants, while one-fifth of the population was enslaved Africans. Before the 1830s，Americans were a relatively homogeneous people in terms of national origin, religion, and physical type. By the third decade of the 19th century, the indigenous peoples had been reduced to subordination, and the only other significant non-European group was black slaves who had been brought to the society beginning in the late 17th century. To accept more immigrants arriving in the new world, the American government chose Ellis Island in 1892 as a special port of entry into New YoA harbor, a port chat has become a historical reputable place, for welcoming millions of immigrants. In 1886, France gave the United States the Statue of Liberty as 这 gesture of friendship, and since then the Statue, which is standing on an island near Ellis Island, has become a symbol of hope, lighting the way for new arrivals. Besides the early settlers and immigrants who took part in building the new nation in the 1700s and 1800s，more than 13 million immigrants came to the United States at the beginning of the 20th century.

Along with the rapid growth of American economy after the Civil War, the demand for more immigrants, most of whom were cheap labors，also increased, and this demand was fulfilled by endless flow of immigrants from Europe tormented by iamine and wars. When cottage industry finally gave way to the process of the Industrial Revolution on both sides of the Atlantic, more people came in search of happiness and new chances for survival. In the mid-1840s, Chinese immigrants, most from impoverished Southeastern China, began to immigrate to the west coast areas near San Francisco and Los Angeles.

Faced with the increasing number of immigrants, American citizens and the gpvemment felt the pressure of living conditions and job market, so they began to ask for restricting the acceptance of unlimited numbers of immigrants. In 1924，Congress passed the Reed-Johnson Immigration Act, setting limits on the influx of newcomers with quotas calculated on nation of origin, which means the number of legible people of a country was based on the number of people firom that country already living in the US. Tliroughout the postwar decades, the policy of nationally based quotas remained effective and the structure of American population considerably reflects this Act even today.

There are two large bodies of immigrants that reflect the particularity of US immigration policy: refugees and illegal immigrants, whose size continues to grow dll now. In 1959 and 1980, the US first acceptcd about 700,000 Cuban refugees and then a group of more than 110,000 Cuban refugees in boats. After the end of the Vietnam War, the US accepted about 750,000 refugees from Indo-China area (the peninsula of S.E. Asia containing Bunna, Thailand, Malaya, Laos，Cambodian and Vietnam). As co the illegal immigrants, the US has more than 4 million people living in the country without legal permission, and many of them have Mexican origin because of historical reasons and the common border with the country. In 1990, about 880,000 people gained legal status and more than 2.5 million people were waiting for being legalized.

In 1965, a new law was passed to ignore immigrants1 country of origin, and immigration patterns thus changed as the US acccpted imm^rants on the basis of who plies first within overall annual limits. This change brou^it about the chained nature of the American population. Today, Black Americans compose about 12.3% of the total population, and there are more than 18 million Hispanic people. The diversity of population has greatly changed people’s concept of "melting-pot" because difTerent groups of people like to live in dbdncdve communities such as "Chinatown" and "Litde Italy' although these people are bound by the same ttAmerican culture".

One problem resulted from immigration is "brain drain", which refers to the immigration to the US of skilled workers, professionals, and technidans who are desperately needed by their home countries. During the mid-twentieth century many scientists and other professionals from industrial nations, principally Germany and Great Britain, came to the US. More recently, however, the brain drain has pulled emigrants from developing countries, including China, India, Pakistan and newly independent African states. This is seen as another symptom of the unequal distribution of world resources.

Legal as well as illegal immigration, along with natural population growth, are rapidly changing the color composition of the United States. The US population consists of more than 300 million people in 2007，including millions of immigrants who have arrived in recent years. At present, minority group members — African Americans，Ladno Americans, Asian Americans and Native Americans 一 together make up about 26% of the US population. By the year 2050, it is estimated that American white will become the minority group.