U.S. travelers flying domestically are required to show a Real ID at airport checkpoints. The May 7 deadline for Americans to switch to the federally compliant identification comes after more than 20 years of being delayed.

Travelers flying within the US starting Wednesday must use a state-issued ID that is “REAL ID compliant” or be subject to further screening. Things seemed to be operating relatively smoothly at a sampling of US airports Wednesday morning.

Those traveling within the United States need only one valid form of identification, according to the TSA’s Real ID FAQ page. Other than a Real ID, the TSA will accept the following alternative forms of identification at security checkpoints following Wednesday’s deadline, which the TSA says are subject to change: