You do not necessarily need a passport to enroll in or use TSA PreCheck, but it is often the most convenient document to use. During the initial enrollment and in-person appointment, you must provide proof of identity and citizenship; an unexpired U.S. Passport serves as both. However, if you do not have one, you can provide a combination of other documents, such as an unexpired U.S. Driver's License (must be REAL ID compliant) and a certified U.S. Birth Certificate. Once you are enrolled and have your Known Traveler Number (KTN), you do not need to carry a passport for domestic flights. At the airport security checkpoint, your KTN is linked to your boarding pass, and you only need to show your standard government-issued photo ID (like a driver's license) to the TSA officer. The "PreCheck" status allows you to leave on your shoes, light jacket, and keep your laptop in your bag, making the screening process significantly faster.