The group are stranded on a ledge deep in the Tham Luang cave but with water rising around them, officials have decided they cannot wait any longer.
Expert divers who have been keeping the group supplied since they were found last week will guide them out.
Officials have called it D-Day, saying the boys are fit enough and ready.
What is happening at the cave
A huge volunteer and media operation has built up around the mouth of the cave over the past week.
But early on Sunday, journalists were told they had to move down the road, sparking speculation that a rescue mission was about to begin.
Narongsak Osottanakorn, who has been leading the operation, then confirmed that 18 divers had gone in to get the boys.
"This is D-Day," he said, saying they had been assessed by a doctor and were "very fit physically and mentally. They are determined and focused."
The group and their families have all given their agreement that they should be moved as soon as possible, he said.
Why are they acting now?
Officials had been exploring whether they could drill down into the cave, as well as scouring the mountainside for another way in.
But with the rainy season just beginning, the flooding which originally trapped the boys will only get worse in the coming days.
Rescuers have been desperately pumping water out of the cave, and the governor said water levels inside were at their lowest levels so far.
"There is no other day that we are more ready than today," said Mr Narongsak. "Otherwise we will lose the opportunity."
How will they bring them out?
This has not been confirmed, but it's likely the group will have to do a mixture of walking, wading, climbing and diving.
The initial part of their journey, through narrow, submerged passages, is the most challenging, the BBC's Jonathan Head reports from near the cave.
It could involve long periods underwater for children who have never used diving equipment before. Some have even had to have swimming lessons inside the cave.
The rescuers plan to move them first to the cavern which has been the forward base for the divers, our correspondent says.
They will then make the last, easier walk out to the entrance, and go from there to hospital in Chiang Rai town.
The earliest the boys are likely to reach the surface is 21:00 local time (14:00 GMT). It's not clear whether they'll all come out at once, or in small groups.
In an indication of quite how dangerous the journey will be, a former Thai Navy diver died in the caves earlier this week. Petty Officer Saman Gunan was returning from a mission to provide the group with air tanks.
He lost consciousness and could not be revived. His colleagues on said "we will not let the sacrifice of our friend go to waste".
Anticipation in the air
By Helier Cheung, Tham Luang cave
This is the moment that everyone has been waiting for - and that some thought would never begin.
Earlier on, the mood around the rescue sites had been upbeat once the boys were found alive. Smiling volunteers would hand out snacks, or offer massages, to those on the site.
Now security to the site has been beefed up even more. Several police vans, military officers and ambulances have been entering and exiting the site. There's a solemn sense of anticipation in the air.
Family and friends of the boys will be getting little rest until they learn the outcome of the rescue operation.
How did they get into cave and what's it like in there?
The boys, all part of the same "Wild Boars football team, are aged between 11 and 16.
It's thought they went in there on 12 June as part of a fun outing to celebrate one of their birthdays, taking in some food.
But it appears they went too far at the wrong time of year, and unexpectedly got cut off.
Against all odds, they were found by rescue divers early last week, perched on a high ledge in the darkness.
They were hungry and scared but in remarkably good shape given their ordeal.
Since then, they've been kept company by navy divers at all times, and had food, light and medical care sent in.
They even managed to send letters home telling their parents not to worry, but that they were looking forward to coming home.