Heathrow's new Terminal 2 has welcomed its first passengers, with the airport insisting it has learned lessons from the opening of Terminal 5.
The first flight, a United Airlines from Chicago, arrived at 05:49 BST.
Passengers on the first of 34 United flights scheduled for Wednesday were greeted by staff in Beefeater costumes.
The airport will open in stages to avoid the chaos in 2008 when Terminal 5 opened and staff struggled to cope with the computer and baggage systems.
The new terminal will operate at 10% of capacity on the first day and it will be some months before the other 25 airlines using Terminals 1, 3 and 4 join United Airlines at the new building.
The opening coincides with a 24-hour strike by members of the RMT union who work on the Heathrow Express rail link to central London.
The union says plans to save £6m from the cost of running the service could affect 200 jobs and hit working conditions, but managers say they plan to run regular trains despite the walk out.
The £2.5bn Terminal 2, known as the Queen's Terminal, replaces the very first passenger building which was opened by the Queen in 1955. Before that, old army tents had been used to house passengers.
Eyes will be on the airport because the opening of Terminal 5 in March 2008 was such a disaster, said the BBC's transport correspondent Richard Westcott.
Staff could not park, find their way around or use new systems. In the end the new luggage system went into meltdown with 15,000 bags stuck in the wrong place, he said.
On the terminal's first day there will be just 34 flights carrying 6,000 passengers. There will eventually be 330 flights a day to 50 destinations by 26 airlines.
The new terminal has 60 check-in gates and 66 self-check-in kiosks, 29 security lanes, 33 shops and 17 restaurants.
There were 178 passengers and 11 crew on the first flight, a Boeing 767.
The UK and Ireland sales director for United Airlines Bob Schumacher said: "We're extremely proud to be the first airline to operate from T2.
"Heathrow is one of the most important airports in United's global network, and T2 represents a huge improvement in the service and facilities we are able to provide to all our customers."
Heathrow development director John Holland-Kaye said: "T2 is the culmination of an £11bn investment programme that has transformed Heathrow for passengers.
"Our measure of success is not everything running perfectly on day one; there will inevitably be things we can improve. Our real measure of success is whether T2 comes to be rated by passengers as one of the world's best airport terminals for years to come."
Passenger Dr Stuart Weinstein, 67, from Iowa City, Iowa, said: "That was the quickest I've ever got through immigration at Heathrow."
Steve Elmore, 47, from Chicago, said: "We saw these guys dressed in Beefeater costumes as we got off the plane. It was a fantastic reception."
Irish student Lee Conneely, 20, from Wexford, who was waiting to board a flight to San Francisco said: "There's not a huge flow of passengers yet, so the test for the terminal will come when it gets busier after a few weeks."
The BBC's transport correspondent said there was a lot at stake because Heathrow wants permission to build a third runway and a new terminal to go with it.