Birthright citizenship is the rule that decides, at the first moment of a life, which government a newborn belongs to and which passport she can one day carry. This UpLaw research article explains the two organizing principles of nationality law—jus soli ("right of the soil") and jus sanguinis ("right of blood")—and shows how virtually every country blends them rather than choosing one. It maps the modern world: unconditional birthright citizenship now survives almost only in the Americas, while the United Kingdom, Ireland, Australia, India, New Zealand, and others have narrowed or abolished pure soil-based rules since 1980, and the Dominican Republic's retroactive 2013 ruling stands as a warning about manufactured statelessness. It then dissects the American guarantee anchored in the Fourteenth Amendment's Citizenship Clause, United States v. Wong Kim Ark, and 8 U.S.C. § 1401(a), and the contested five words—"subject to the jurisdiction thereof"—now driving a 2025 executive order that, as of this writing, remains blocked and pending before the Supreme Court. Worked hypotheticals, primary and secondary authority, and a candid policy ledger round out a treatment written for lawyers and non-lawyers alike, with careful attention to what the law does and does not currently settle.