The lineup of teams at the World Cup in Russia will be confirmed over the next week.
Here's a look at the two-leg European and intercontinental playoffs matches, as well as how African qualifying is shaping up:
NORTHERN IRELAND vs. SWITZERLAND
Northern Ireland is within reach of its first World Cup in 32 years after hitting its target of second place in a group won by defending champion Germany.
Switzerland, ranked No. 4 in August, finds itself in a tense playoff despite nine straight wins to start qualifying.
On Monday, Switzerland coach Vladimir Petkovic was still looking back to the 2-0 loss at Portugal last month, saying he had imagined playing a "nice friendly" game this week. He acknowledged his team was too passive against Portugal, and challenged players he did not identify to take more responsibility now.
The Swiss need to forget about that match before the first leg Thursday at Windsor Park, where only Germany has beaten Northern Ireland in a competitive match in the past four years.
Switzerland has a nine-game winning streak at home in tournament qualifiers, and hosts the return on Sunday in Basel.
CROATIA vs. GREECE
Greece is bidding to reach a major tournament again after an embarrassing qualification campaign for the 2016 European Championship.
Without defender Kostas Manolas, who is banned from the first leg, the Greeks will play at Croatia on Thursday and then host the return match on Sunday.
Manolas is out because FIFA suspended him for trying to manipulate his disciplinary record by deliberately earning a yellow card against Cyprus on Oct. 7. The decision compounded problems for coach Michael Skibbe, who is missing midfielder Petros Mantalos and striker Tasos Donis because of injury.
Croatia is set to miss defensive midfielder Milan Badelj for both matches against Greece.
The Croatians and the Greeks were in the playoffs for the 2014 tournament in Brazil. Croatia eliminated Iceland 2-0 on aggregate, while Greece beat Romania 4-2 on aggregate.
SWEDEN vs. ITALY
Four-time champion Italy has participated in every World Cup since failing to qualify for the 1958 tournament, which coincidentally was held in Sweden.
But Italy has showed poor form recently, ever since a 3-0 loss to Spain in September. After that match, Italy edged Israel 1-0, drew 1-1 with Macedonia and beat Albania 1-0.
While Sweden didn't qualify for the last two World Cups, it impressed in qualifying with a win over France and finished ahead of the Netherlands.
"We are winners just by being here," Sweden coach Janne Andersson said.
The first leg will be in Sweden on Friday, with the return match on Monday in Italy.
Sweden went unbeaten at home in qualifying at Friends Arena and striker Marcus Berg scored eight times in nine matches.
DENMARK vs. IRELAND
Being in the playoffs for a major tournament is nothing new for Ireland.
This will be the team's ninth time, and they have lost five of them - including against France ahead of the 2010 World Cup when Thierry Henry clearly handled the ball in the build-up to the crucial goal.
The first leg will be in Denmark on Saturday. Ireland will host the second leg on Tuesday.
Ireland has effectively already won one playoff - by beating Wales 1-0 in its final qualifier - and will likely rely on the counterattacking tactics that worked so well in Cardiff.
While there are no real stars in the Ireland team, Denmark has a standout in Christian Eriksen - one of European soccer's best playmakers over the past few years at Tottenham. He scored eight times from midfield in qualifying.
The Danes' 4-0 win over Poland in group play highlights their potential and they are also higher in the FIFA rankings, at No. 19 compared to Ireland's No. 26.
The managers - Martin O'Neill and Age Hareide - are former teammates at English club Norwich.
AUSTRALIA vs. HONDURAS
This is the scenario Australia aimed to avoid when it switched from Oceania to the Asian Football Confederation after the 2006 World Cup: A last-ditch intercontinental playoff in the Americas for a spot at soccer's marquee event.
Australian players traveled from three continents to assemble in Honduras this week to prepare for Friday's first leg. The return match will be in Australia on Wednesday.
It was a similar scenario when the Australians played Uruguay for a place at the 2006 tournament. Uruguay won 1-0 in Montevideo, and Australia leveled it up with a 1-0 win in Sydney before advancing 4-2 on penalties. It ended a long drought for the Socceroos, who had failed to advance from the last playoff stage in 1986 (against Scotland), in 1994 (against Argentina), in 1998 (against Iran), and in 2002 (against Uruguay).
Tim Cahill, the only player still active from that playoff series in 2005 and now Australia's all-time leading scorer, is in doubt because of an ankle injury.
NEW ZEALAND vs. PERU
New Zealand will return to the scene of one of its greatest triumphs when it hosts Peru on Saturday.
In 2009, New Zealand beat Bahrain 1-0 in front of a crowd of 35,000 at the Wellington Regional Stadium to qualify for the World Cup for the second time. It had previously qualified in 1982, the same year in which Peru made its last World Cup appearance.
The return leg will be in Peru on Wednesday.
Rory Fallon, who scored New Zealand's winning goal in that match eight years ago, has been recalled to the All Whites after a one-year absence. He is currently playing for Dorchester in the seventh tier of English soccer. That win over Bahrain briefly put soccer center-stage in a rugby-mad country and Saturday's match will likely do so again. The match is a sellout.
Peru, ranked No. 10, starts as the overwhelming favorite against 122nd-ranked New Zealand but have a significant setback, with striker Paolo Guerrero ruled out after failing a doping test. Guerrero scored six goals in qualifying.
Africa's last three World Cup places will be decided on the final group games in qualifying, when Ivory Coast could miss out and Congo could qualify for the first time since 1974.
Two of the five groups are settled, with Nigeria and Egypt already through.
Tunisia and Congo will tussle for a place from Group A. Tunisia has a three-point advantage, so Congo needs to beat Guinea and hope Tunisia loses at home to Libya. If that happens, the World Cup place will go to the team with the best goal difference.
Morocco and Ivory Coast face a straight shootout in Group C when Morocco travels to Abidjan. Ivory Coast must win to sneak through to the World Cup and deny former coach Herve Renard, now with Morocco.
Senegal can seal its place on Friday in a contentious qualifier against South Africa, a game ordered by FIFA to be played again because of match-fixing by the referee. Senegal lost the initial game but if it wins on its second chance, it'll be through to the World Cup.
That will lead to serious discontent from the other teams. Burkina Faso, hoping to go to the World Cup for the first time, has already appealed against FIFA's decision.