CAT scan machines detect explosives chemically
From now on, all luggage checked into the hold of a plane must be screened.
But BBC Washington correspondent Rob Watson says the new measures are already causing significant delays for passengers waiting to check in.
And there are concerns that the checks do not go far enough and would not thwart a determined bomber.
Delays
US airports annually load about 14 billion pieces of luggage onto planes but until now less than 10% of that has been screened in any way.
Now, under the new legislation every bag must be checked either by sniffer dogs, bomb detection machines using CAT (computerised axial tomography) scan technology, by hand or at the very least by matching it to a travelling passenger.
As very few machines or dogs are yet available, most airlines will simply make sure passengers and their luggage travel together.
As our Washington correspondent Rob Watson reports, at Washington's Reagan Airport the checks are already causing huge passenger queues.
And while so far, most travellers have heeded the government's call for patience as a form of patriotism, it is unclear how long that patience will last in a country where people are used to catching a flight in the way other people catch a bus.
Experts say this is a cause for some concern among the airlines when passenger traffic is down a massive 20% since 11 September.
Insufficient measures
But critics say the checks will do nothing to deter a suicide bomber.
"The initiative is still with the terrorists," said David Forbes, president of aviation security consortium BoydForbes Inc.
Others, notably in Congress, say the measures do not go far enough because they do not apply to every passenger on every flight.
"Under the administration's plan, only under rare circumstances will officials actually determine what is inside checked baggage as the law requires," said House Democratic leader Richard Gephardt.
But the new measures are only the beginning - in February, the government will take over running security at all of America's airports.