It's Day One of parenting for William and Kate. After the excitement and fatigue and joy of childbirth - emotions shared with a nation - the young couple is expected to bring the prince home and start to care for him.
But the lad's name - and his likeness - remain a royal mystery. And as the infant child begins a long journey expected to see him someday become a king, Brits - and people from around the world - have been joining the royal family in celebration.
"The whole country will celebrate," Prime Minister David Cameron said, paying tribute to Prince William and his wife, Kate. "They'll make wonderful parents."
After an impromptu party at Buckingham Palace, more celebrations are expected Tuesday, including gun salutes by royal artillery companies to honor the birth. Riders in uniform will trot past the palace to Green Park, where six field guns will fire 41 blank rounds.
Halfway around the world, royalist group Monarchy New Zealand said it had organized a national lightshow, with 40 buildings across the island lit up in blue to commemorate the royal birth, including Sky Tower in Auckland, the airport in Christchurch, and Larnach Castle in the South Island city of Dunedin.
The official announcement of the birth of the Duke and Duchess of Cambridge's son. (Photo courtesy: @thebritishmonarchy/Twitter)
The baby isn't even a day old - and may not even been named for days or even weeks - but he already has a building dedicated to him.
In Australia, Prime Minister Kevin Rudd said an enclosure at Sydney's Taronga Park Zoo would be named after the prince as part of a gift from Australia. The government would donate 10,000 Australian dollars ($9,300) on the young prince's behalf toward a research project at the zoo to save the endangered bilby, a rabbit-like marsupial whose numbers are dwindling in the wild. The prince's name - when known - would be added to the bilby enclosure.
"I don't know if the royal family would need this, but we'll probably give them a free pass to Taronga Park Zoo as well," Rudd said.
British media joined in the celebration.
"It's a Boy!" was splashed across many U.K. front pages, while Britain's top-selling The Sun newspaper temporarily changed its name to "The Son" in honor of the tiny monarch-in-waiting.
"REGAL HAS LANDED," the paper cried.