From PBS.Org:
Dred Scott first went to trial to sue for his freedom in 1847. Ten years later, after a decade of appeals and court reversals, his case was finally brought before the United States Supreme Court. In what is perhaps the most infamous case in its history, the court decided that all people of African ancestry — slaves as well as those who were free — could never become citizens of the United States and therefore could not sue in federal court. The court also ruled that the federal government did not have the power to prohibit slavery in its territories. Scott, needless to say, remained a slave.
Born around 1800, Scott migrated westward with his master, Peter Blow. They travelled from Scott’s home state of Virginia to Alabama and then, in 1830, to St. Louis, Missouri. Two years later Peter Blow died; Scott was subsequently bought by army surgeon Dr. John Emerson, who later took Scott to the free state of Illinois. In the spring of 1836, after a stay of two and a half years, Emerson moved to a fort in the Wisconsin Territory, taking Scott along. While there, Scott met and married Harriet Robinson, a slave owned by a local justice of the peace. Ownership of Harriet was transferred to Emerson.
Scott’s extended stay in Illinois, a free state, gave him the legal standing to make a claim for freedom, as did his extended stay in Wisconsin, where slavery was also prohibited.
The decision of the court was read in March of 1857. Chief Justice Roger B. Taney — a staunch supporter of slavery — wrote the “majority opinion” for the court. It stated that because Scott was black, he was not a citizen and therefore had no right to sue. The decision also declared the Missouri Compromise of 1820, legislation which restricted slavery in certain territories, unconstitutional.
More here.