The Underground Railroad was an organized network of secret routes and safe houses used by freedom seekers to escape to the abolitionist Northern United States and Eastern Canada. Slaves escaped from slavery as early as the 16th century; many of their escapes were unaided. However, a network of safe houses generally known as the Underground Railroad began to organize in the 1780s among Abolitionist Societies in the North. It ran north and grew steadily until President Abraham Lincoln issued the Emancipation Proclamation in 1863. The escapees sought primarily to escape into free states, and potentially from there to Canada.

The Underground Railroad started at the place of enslavement. The routes followed natural and man-made modes of transportation: rivers, canals, bays, the Atlantic Coast, ferries and river crossings, roads and trails. Locations close to ports, free territories and international boundaries prompted many escapes.

The network, primarily the work of free and enslaved African Americans, was assisted by abolitionists and others sympathetic to the cause of the escapees. The slaves who risked capture and those who aided them were collectively referred to as the passengers and conductors of the Railroad, respectively. Various other routes led to Mexico, where slavery had been abolished, and to islands in the Caribbean that were not part of the slave trade. An earlier escape route running south toward Florida, then a Spanish possession (except 1763–1783), existed from the late 17th century until approximately 1790. During the American Civil War, freedom seekers escaped to Union lines in the South to obtain their freedom. One estimate suggests that by 1850, approximately 100,000 slaves had escaped to freedom via the network. According to former professor of Pan-African studies J. Blaine Hudson, who was dean of the College of Arts and Sciences at the University of Louisville, by the end of the Civil War, 500,000 or more African Americans had self-emancipated from slavery on the Underground Railroad.

Eric Foner wrote that the term "was perhaps first used by a Washington newspaper in 1839, quoting a young slave hoping to escape bondage via a railroad that 'went underground all the way to Boston'". Dr. Robert Clemens Smedley wrote that following slave catchers' failed searches and lost traces of fugitives as far north as Columbia, Pennsylvania, they declared in bewilderment that "there must be an underground railroad somewhere," giving origin to the term. Scott Shane wrote that the first documented use of the term was in an article written by Thomas Smallwood in the August 10, 1842, edition of Tocsin of Liberty, an abolitionist newspaper published in Albany. He also wrote that the 1879 book Sketches in the History of the Underground Railroad said the phrase was mentioned in an 1839 Washington newspaper article and that the book's author said 40 years later that he had quoted the article from memory as closely as he could.

Members of the Underground Railroad often used specific terms, based on the metaphor of the railway. For example:

The Big Dipper (whose "bowl" points to the North Star) was known as the "drinking gourd". The Railroad was often known as the "freedom train" or "Gospel train", which headed towards "Heaven" or "the Promised Land", i.e., Canada.

Many of the fugitive slaves who "rode" the Underground Railroad considered Canada their final destination. An estimated 30,000 to 40,000 of them settled in Canada, half of whom came between 1850 and 1860. Others settled in free states in the north. Thousands of court cases for escaping fugitive slaves were recorded between the Revolutionary War and the Civil War. Under the original Fugitive Slave Act of 1793, officials from free states were required to assist slaveholders or their agents who recaptured fugitives, but some state legislatures prohibited this. The law made it easier for slaveholders and slave catchers to capture African Americans and return them to slavery, and in some cases allowed them to enslave free blacks. It also created an eagerness among abolitionists to help enslaved people, resulting in the growth of anti-slavery societies and the Underground Railroad.

With heavy lobbying by Southern politicians, the Compromise of 1850 was passed by Congress after the Mexican–American War. It included a more stringent Fugitive Slave Law; ostensibly, the compromise addressed regional problems by compelling officials of free states to assist slave catchers, granting them immunity to operate in free states. Because the law required sparse documentation to claim a person was a fugitive, slave catchers also kidnapped free blacks, especially children, and sold them into slavery. Southern politicians often exaggerated the number of escaped slaves and often blamed these escapes on Northerners interfering with Southern property rights. The law deprived people suspected of being slaves of the right to defend themselves in court, making it difficult to prove free status. Some Northern states enacted personal liberty laws that made it illegal for public officials to capture or imprison former slaves. The perception that Northern states ignored the fugitive slave laws and regulations was a major justification offered for secession.