Beginning January 22, 2018, ALL passengers boarding an aircraft in the United States for domestic travel may be required to present a REAL ID driver’s license or valid U.S. passport.
Some states have not complied with offering the REAL ID, meaning people living in certain states can ONLY use a valid U.S. passport for domestic travel starting on Jan 22, 2018. You need to check with your state on their current status, restrictions and exemptions for travel.
For International travel, as always, you must have a valid U.S. passport good for at least 6 months (sometimes 7) beyond your return date of travel. U.S. Passport Information.