Indian tribes are separate sovereigns — not subdivisions of the states, not federal agencies, and not private associations — and almost every hard question in federal Indian law flows from that single premise. This UpLaw guide traces the doctrine from the Marshall Trilogy's "domestic dependent nations" through congressional plenary power, the federal trust responsibility, and the Indian-law canons of construction, then works through the questions practitioners actually face: what "Indian country" means under 18 U.S.C. § 1151 and why that definition decides everything; how reservation-diminishment analysis works after McGirt v. Oklahoma and what Oklahoma v. Castro-Huerta did and did not change; the criminal-jurisdiction matrix built from the Major Crimes Act, the General Crimes Act, Oliphant, Public Law 280, and the Violence Against Women Act's special tribal criminal jurisdiction; and the steadily narrowing civil authority of tribes over nonmembers under Montana, Strate, Hicks, and Plains Commerce Bank. It then covers tribal sovereign immunity in depth — its default breadth after Kiowa and Michigan v. Bay Mills, the arm-of-the-tribe test, the individual-capacity limit of Lewis v. Clarke, and what a genuine waiver must say — along with taxation, the Indian Child Welfare Act after Haaland v. Brackeen, gaming under IGRA, allotment and fee-to-trust after Carcieri, and the practical mechanics of contracting with a tribal enterprise. A running hypothetical follows a fabrication vendor that signs with a tribal development authority and discovers immunity only after the invoices go unpaid. Every tribe has its own constitution, code, and courts; this guide teaches the federal framework, not any particular tribe's law.