The Fourteenth Amendment says that people born in the United States and subject to its jurisdiction are citizens. For more than a century that has been read to cover nearly everyone born on American soil. Whether it must be read that way is now being litigated.
The amendment was adopted in 1868 to overturn the Dred Scott decision and secure citizenship for freed slaves. Its citizenship clause has two requirements: birth in the United States, and being subject to its jurisdiction.
In 1898 the Supreme Court held in United States v. Wong Kim Ark that a child born on American soil to Chinese parents who were themselves ineligible for citizenship was nonetheless a citizen. That decision has been the operative reading ever since, and administrative practice has followed it without interruption.
The narrow historical exceptions to jurisdiction are children of accredited foreign diplomats, children born to a hostile occupying force, and, until federal legislation resolved it, some children born on tribal land. Whether the children of unauthorized immigrants or temporary visitors could be added to that list is the contested question.
Most of the world's countries do not grant citizenship by birth alone; unrestricted birthright citizenship is most common in the Americas. Comparisons cut both ways in the debate, since other countries' rules rest on different constitutional texts and histories.