Northern Ireland joined England and Wales in the last 16 of the European Championship after finishing as one of the best third-placed teams.
Making their first appearance at a Euros, they will meet Wales or hosts France for a place in the last eight.
Michael O'Neill's side were beaten 1-0 by world champions Germany in their final Group C game on Tuesday.
But their tally of three points was enough to claim third spot and a place in the next round on goal difference.
With the groups yet to be completed, Northern Ireland are already assured of being one of the four best third-placed teams to progress to the knockout stages alongside the group winners and runners-up.
They will learn whether they will face Wales or France once Wednesday's group games are over.
A tie against Wales would take place on Saturday (17:00 BST) at the Parc de Princes in Paris, while they would meet France in Lyon on Sunday (14:00).
"It's surreal," said O'Neill, who revealed his squad found out they had qualified by following the BBC Sport website.
"What we saw was a team that refused to be beaten by a big margin and did everything possible to stay in the game.
"I'm not sure it's really sunk in yet, but it's a fantastic achievement. Over the three games, we deserve to be in the last 16.
"What we went through tonight will prepare us for anything in the next round and our players will be ready for whoever."
Asked whether he would rather face Wales or France, O'Neill added: "If I was picking a team I thought we had a better chance against, I would say Wales."
Who could England face?
Roy Hodgson's side will play their last-16 match on Monday at 20:00 BST against the side that finishes second in Group F.
That could be Hungary, Iceland, Portugal or Austria, who play their final round of group games on Wednesday..
By failing to finish top of Group B, England now find themselves in the same side of the draw as Spain, Italy, Germany, France and, potentially, Portugal.
What about Wales?
Chris Coleman's team, who finished above England in Group B, could face Northern Ireland or Turkey.
Whatever happens, the game will take place on Saturday (17:00 BST) in Paris,
Other teams in their half of the draw include Switzerland, Poland and Croatia.
Who else has qualified?
We know 12 of the 16 qualifiers ahead of Wednesday's final round of group games:
Group A: France and Switzerland
Group B: Wales and England
Group C: Germany and Poland
Group D: Croatia and Spain
Group E: Italy
Group F: Hungary
Best third-placed teams: Slovakia and Northern Ireland
Who else can go through?
Two further places are up for grabs in Groups E and F.
A further two best third-placed spots are available, with Albania and Turkey still in with a chance of making it through.
The Republic of Ireland will qualify for the last 16 if they beat Italy on Wednesday (20:00 BST).
Italy are set to make "between seven and nine" changes for the game, according to boss Antonio Conte.
The Italians already know they will face Spain in the last 16 after the defending champions finished second in Group D following a surprise loss to Croatia.
Who is out?
Eight teams will fail to progress to the last 16. We know four of them.
Romania, Russia, the Czech Republic managed just one point each, while Ukraine failed to collect a goal or point.