The group are stranded on a ledge 4km inside the Tham Luang cave but amid fears of rising waters, officials have decided they cannot wait any longer.
Officials have called it D-Day, saying the boys are fit and ready to move.
Expert divers will help guide them out but authorities stress they do not know how long it will take.
The boys are expected to be brought out in groups, however a statement from the Thai government said they were yet to decide how many will emerge in the first operation.
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. “The boys are ready to face any challenges.”
He added that the boys had all been assessed by a doctor and were “very fit physically and mentally… They are determined and focused”.
The group and their families had all given their agreement that they should be moved as soon as possible, he said.
Why are they acting now?
Officials had originally thought the group might have to stay where they were until the rainy season ended – that could have meant months underground.
They’d also 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, it’s become clear that 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 Mr Narongsak said on Sunday that 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?
Getting to and from where the boys are has been an exhausting 11-hour round trip even for the experienced divers.
When the operation began, officials said the earliest the boys were likely to reach the surface was 21:00 local time (14:00 GMT) on Sunday. One official has said it could take two to three days to get them all out.
They’ll have to do a mixture of walking, wading, climbing and diving – all in complete darkness – along guide ropes already in place.
Wearing full-face masks, which are easier for novice divers than traditional respirators, each boy will be accompanied by two divers who will also carry their air supply.
The toughest section is about halfway out – they’ll reach section called “T-Junction”, which is so tight the divers will have to take off their air tanks to get through.
Eventually they’ll reach Chamber 3, the cavern which has been turned into a forward base for the divers. They’ll rest there before making the last, easier walk out to the entrance. They’re expecting to be taken straight to hospital in Chiang Rai town.
In an indication of quite how dangerous the journey will be, a former Thai navy diver died in the caves earlier this week. Saman Gunan was returning from a mission to provide the group with air tanks.
He lost consciousness and could not be revived. His colleagues have said they “will not let the sacrifice of our friend go to waste”.
Sense of anticipation
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.
Why did they go into the cave?
The boys, all part of the same Wild Boars football team, are aged between 11 and 17 and know the local area well.
It’s thought they went in there on 12 June after training as a fun outing, to celebrate one of the boy’s birthdays. They took in only basic food.
But it appears they went in 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.