The tournament will consist of 32 teams, with hosts Russia joined by 31 qualifiers from around the globe. The holders no longer qualify automatically, although Germany are on track to progress.
Eight sides are already guaranteed to be there: Russia, Brazil, Iran, Japan, Mexico, Belgium, Saudi Arabia and South Korea.
In Europe, the country that tops each of the nine groups reaches the finals, while the eight best second-placed sides go into play-offs.
What about the home nations?
Group C - Thursday: Germany (h) 19:45, Sunday: Norway (a) 19:45 (all times BST)
With just one defeat and only two goals conceded in eight matches, Northern Ireland - second in Group C - have more points than some group leaders.
Nine clear of third-placed Azerbaijan, they are guaranteed a top-two place as they look to reach their first World Cup since 1986.
Automatic qualification seems unlikely though - they need to beat Germany and Norway, and hope the Germans lose their final match against Azerbaijan. The reigning champions need just a draw in Belfast to top the group.
Group D - Friday: Georgia (a) 17:00, Monday: Republic of Ireland (h) 19:45
"Don't take me home, please don't take me home," sang the fans as Walesmarched to the semi-finals of Euro 2016.
Now they want to cheer them in a second World Cup, with their only previous finals appearance coming in 1958. However, Chris Coleman's side must play their final two group matches without injured talisman Gareth Bale.
A win in Georgia on Friday will be enough to secure a top-two spot if the Republic of Ireland fail to beat Moldova at home. Otherwise, it could come down to Monday's meeting between Wales and Martin O'Neill's side in Cardiff.
Serbia lead the group and will qualify with a victory in Austria or at home to Georgia.
The Republic of Ireland can enter the play-off picture by defeating both Moldova and Wales. A win over Moldova and a draw with Wales would only be enough if the Welsh fail to beat Georgia.
Group F - Thursday: Slovenia 19:45 (h), Sunday: Lithuania (a) 17:00
Unbeaten England, with six wins and two draws, comfortably lead Group F and are poised to qualify automatically. They require one win, or two draws, or just a single point if second-placed Slovakia do not win both their matches.
Group F - Thursday: Slovakia 19:45 (h), Sunday: Slovenia (a) 17:00
With just four points from their first four matches, few were rushing to back Scotland to reach Russia.
But victory over Slovenia, a draw with England and wins against Lithuania and Malta have put them back in the hunt for a first World Cup appearance since 1998.
Scotland could reach the play-offs by winning their final two matches, but defeat by Slovakia will end their hopes. Realistically, they need to beat Slovakia, because they will surely take maximum points in their final match at home to Malta.
The play-off puzzle
There's no easy way of explaining this, but here goes...
Eight of the nine European runners-up go into a two-legged play off for four places at next year's finals. That means the runner-up with the fewest points misses out. That unfortunate position is currently occupied by Wales, although plenty can change over the coming days.
Bosnia-Herzegovina, for example, are also on eight points and face two difficult games against already-qualified Belgium and an away game in Estonia.
It is important to note that results gained against the bottom-ranked sides in each group do not count in the final play-off standings.
This aspect also affects Scotland - who do not feature in the table below because they are currently fourth in their table - because having beaten Malta twice, like Wales they are effectively only on eight points.
The ranking of second-placed teams is decided simply by the highest number of points but then - if required - goal difference, followed by the highest number of goals scored, the highest number of goals scored away from home and then fair play points.
Fifa says teams will be seeded for the draw on 17 October, with the top four sides according to its ranking at the time in one pot and the remaining four in another.
The teams currently in second place are Portugal (3rd in Fifa rankings), Wales (13th), Italy (17th), Slovakia (19th), Northern Ireland (20th), Iceland (22nd), Sweden (23rd), Bosnia-Herzegovina (36th) and Montenegro (37th).
So while there are plenty of permutations, a Wales-Northern Ireland play-off is a possibility.
2 N Ireland
Rest of Europe - could the Netherlands miss out?
Europe's places: 14 including hosts Russia. Already qualified: Belgium, Russia
They may be three-times World Cup runners-up but the Netherlands are in a scrap to reach next year's tournament.
The Dutch are in third place in Group A, behind France and Sweden, and will be out if they fail to at least match the Swedes' result on Saturday.
Sweden host Luxembourg before travelling to the Netherlands - who first visit Belarus - in a potential decider on Tuesday.
Surprise Euro 2016 quarter-finalists Iceland, who were the smallest nation to qualify for a major finals when they reached France two years ago, are in second place but level on points with Group I leaders Croatia.
Victories over Turkey and Kosovo would secure a play-off place at least, although the Turks and Ukraine are just two points behind.
Portugal, while sure of at least making the play-offs, face a critical final match at home to Switzerland on Tuesday as Cristiano Ronaldo and his team-mates try to overturn a three-point deficit in Group B.
Road to Russia: The key dates
17 October: Europe play-off draw (Zurich)
9-11 November: Play-off first legs
12-14 November: Play-off second legs
1 December: The groups are drawn
14 June-15 July 2018: World Cup finals
Asia - The Syria story
Asian places: 4 (plus a play-off place). Already qualified: Iran, South Korea, Japan, Saudi Arabia
Now this is some tale.
Syria remain in contention for the World Cup despite the odds being stacked against a nation that is in the middle of a six-year war.
The team has no funding because of sanctions and play their home games in Malaysia, which is a 9,000-mile round trip.
But they have made it through to a play-off against Australia and Omar Al Somah's late penalty allowed them to claim a 1-1 first-leg draw.
The return leg takes place in Sydney on Tuesday, 10 October (kick-off 10:00 BST).
The winner will face a Concacaf (North and Central America and the Caribbean) side in another two-legged tie the following month for a place at the World Cup.
South America places: 4 (plus a play-off place). Already qualified: Brazil
It seems almost inconceivable that Lionel Messi will not be at football's biggest tournament, but his Argentina side are in real danger of missing out.
The two-time champions, who have not missed the finals since 1970, are currently languishing in fifth place.
With the top four qualifying, that position would send them into a two-legged play-off against New Zealand, but even that scenario is far from assured with Copa America holders Chile lurking just a point behind in sixth.
Argentina take on Peru, who are above them in fourth on goal difference, before facing Ecuador.
North America, Central America and Caribbean places: 3 (plus a play-off place). Already qualified: Mexico.
Mexico will compete at their seventh straight World Cup next summer, with Costa Rica also on the verge of direct qualification from the Concacaf section.
But a third automatic berth is still up for grabs, while the fourth-placed side can also qualify through the inter-continental play-offs.
Surprise package Panama, in third, lead the United States and Honduras by a single point.
North America, Central America and Caribbean World Cup qualifying: Fixtures - Tables
Africa - Nigeria on track
African places: 5 Already qualified: None
Nigeria are closing on a sixth World Cup appearance while Egypt - led by Liverpool's Mohamed Salah - are on course for their first finals since 1990, but Africa Cup of Nations champions Cameroon are out of the running.
There are no play-offs in Africa, with the team that tops each of the five qualifying groups going through. Tunisia, Ivory Coast and Burkina Faso are the other current group leaders.
Oceania places: One play-off place Already qualified: None
New Zealand beat the Solomon Islands over two legs (as winners of their respective groups) to book their place in the inter-continental playoffs, and will take on the fifth-placed team from South American qualifying in November.