In the context of the 20th-century history of the United States, the Second Great Migration was the migration of more than 5 million African Americans from the South to the Northeast, Midwest, and West. It began in 1940, through World War II, and lasted until 1970.  It was much larger and of a different character than the first Great Migration (1916–1940), where the migrants were mainly rural farmers from the South and only came to the Northeast and Midwest.
In the Second Great Migration, not only the Northeast and Midwest continued to be the destination of more than 5 million African Americans, but also the West as well, where cities like Los Angeles, Oakland, Phoenix, Portland, and Seattle offered skilled jobs in the defense industry. Most of these migrants were already urban laborers who came from the cities of the South. In addition, African Americans were still treated with discrimination in parts of the country, and many sought to escape this. 
Compared to the more rural migrants of the period 1910–1940, many African Americans in the South were already living in urban areas and had urban job skills before they relocated. They moved to take jobs in the burgeoning industrial cities in the North and West, including the defense industry during World War II.  Workers who were limited to segregated, low-skilled jobs in some cities were able to get highly skilled, well-paid jobs at California, Oregon, and Washington shipyards. 
By the end of the Second Great Migration, African Americans had become a highly urbanized population. More than 80% lived in cities, a greater proportion than among the rest of American society. 53% remained in the Southern United States, while 40% lived in the Northeast and North Central states and 7% in the West. 
While African Americans were often relegated to support roles during World War II, often these roles could be exceedingly hazardous. A munitions explosion at Port Chicago, California, across the bay from San Francisco, claimed the lives of over 200 African Americans in 1944. When some of the workers refused to resume work until conditions were made less hazardous, up to 50 were tried for mutiny and imprisoned. 
Like the first Great Migration, African American southerners were motivated to move based on economic concerns. The economic deprivation suffered by southern African Americans prior to the Great Migration has been well documented. Plantation agriculture such as sharecropping limited African Americans to gain and work up the "agricultural ladder".  The agricultural ladder was a system in which a worker's status determined their position, not how skilled or valuable they were. Because of segregation, African American men were placed in unskilled jobs and women were placed in domestic services. These conditions had little to no change from the early decades of the twentieth century, which was a powerful incentive for African American southerners to leave and go look for opportunity elsewhere.
In the 1930s and 1940s, the introduction of the mechanical cotton picker and other forms of farm mechanization reduced the demand for unskilled labor in southern agriculture, leading many African Americans to seek new jobs in urban areas.  African Americans strove to find new opportunities such as finding better ways to earn a living to support their families. Migrating north and west out of the South was a way to improve their economic fortunes. World War II resulted in labor shortages due to millions of people enlisting or being drafted into active military service. As a result, Northern and Western employers began recruit southern blacks and whites to keep up with the nation's demands for the war effort. Other social factors such as education opportunities, political disenfranchisement, and racial violence also drove migration. The Great Migration was a complex process driven by a large and diverse set of forces. 
The rapid mobilization of resources and weapons during World War II prompted many African Americans to migrate to Northern and Western cities in search of jobs in the booming munitions industry. While the Northern black communities such as Chicago and New York City were already well-established from the first Great Migration, relocating to the West was a new destination for the migrants in places like the San Francisco Bay Area, Los Angeles, Portland, Phoenix, and the Puget Sound region in Washington. Once they arrived, they were met with a variety of factors that separated emerging migrant communities from white ones. This urban spatial segregation led to the creation of racially homogeneous areas in cities that saw large amounts of African American migration. It is estimated that less than 1% of Los Angeles's 461,000 black residents lived in communities without a black majority in 1960, resulting in de facto segregation. 
In order to exploit the poor financial situation many migrants were in, areas of low income housing were established in places city planners wanted them to live. For example, the South Side area of Chicago and the South Central region of Los Angeles were established as designated areas for African Americans as early as the 1920s and 1930s respectively. Working class blacks were attracted by the low price of housing intentionally placed in order to encourage the concentration of minorities away from whites. Freeways were often built with the isolation of racial communities in mind. They served to reinforce lines of segregation and further contributed to the isolation of racially homogeneous communities in racially diverse cities. The fear of racially motivated violence and discrimination also served to isolate communities of minorities as they sought collective security and nondiscriminatory treatment close to home. The limited amount of housing, combined with the upsurge of migrants during the Second Great Migration lead to severe overcrowding and housing shortages that contributed to increasingly low property values.
As property values within these communities plummeted, the middle class, mostly white residents of target areas evacuated en masse. This is a phenomenon known as white flight. The introduction of non-white residents into a traditionally white area, in this case, inner city neighborhoods, caused the rapid evacuation of whites, usually to the suburbs. In this way, the common social trope of white suburbia and the racially diverse inner city came to be. It is currently estimated that over 70% of blacks operating in a given metropolitan area live in the city center. Comparatively, only 30% of whites operating within that same city live in the inner city.  White flight was driven in part by the process of blockbusting. White property owners, fearful of minority groups, sold their homes to real estate agents at a low price, often due to the tactics of the real estate companies themselves. Agents would then encourage that the vacant properties be bought by black families seeking respite from the overcrowded neighborhoods in which they were sequestered. When one black family moved in, the white neighbors would immediately sell their homes to the waiting real estate companies, who would in turn sell to more black people at a significant markup.
Another obstacle facing migrating blacks were the discriminatory housing laws that were put in place to counteract progressive legislation following World War II. The 1963 Rumford Fair Housing Act that outlawed discrimination in housing was effectively nullified by California Proposition 14 in 1964. This legislation, sponsored by the California Real Estate Association and conservative state institutions, affirmed the property owner’s right to refuse to sell, rent or lease their property based solely on race. The federal government immediately cut funding to California housing and both the California and United States Supreme Court declared Prop 14 unconstitutional. It was officially repealed by California Proposition 7 in 1974, but while it was in practice it contributed to the segregation of black migrants and is cited as a direct cause of the Watts riots in 1965.
The effect of racially homogeneous communities composed largely of migrant blacks that formed because of spatial segregation in destination cities was that they were largely influenced by the Southern culture they brought with them. The food, music and even the discriminatory white police presence in these neighborhoods were all imported to a certain extent from the collective experiences of the highly concentrated African American migrants. 
- "In Motion: African American Migration Experience, The Second Great Migration". Retrieved 2007-03-18.
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- Gibson, Campbell; Kay Jung (September 2002). Historical Census Statistics on Population Totals By Race, 1790 to 1990, and By Hispanic Origin, 1970 to 1990, For The United States, Regions, Divisions, and States. Working Paper Series No. 56. Washington, DC: Population Division, U. S. Census Bureau. Archived from the original on 2008-07-25. Retrieved 2013-05-21.
|Booknotes interview with Nicholas Lemann on The Promised Land, May 5, 1991, C-SPAN|
|Q&A interview with Isabel Wilkerson on The Warmth of Other Suns, September 26, 2010, C-SPAN|
- Arnesen, Eric. Black Protest and the Great Migration: A Brief History with Documents (2002).
- Gregory, James. The Southern Diaspora: How the Great Migrations of Black and White Southerners Transformed America. (University of North Carolina Press, 2005).
- Grossman, James R. Land of Hope: Chicago, Black Southerners, and the Great Migration (1991).
- Lemann, Nicholas. The Promised Land: The Great Black Migration and How It Changed America (1991)
- Sernett, Milton. Bound for the Promised Land: African Americans' Religion and the Great Migration (1997).
- Wilkerson, Isabel. The Warmth of Other Suns: The Epic Story of America's Great Migration (Random House, 2010) 640pp; based on over 1000 interviews; ISBN 978-0-679-44432-9