Arsenal and Bayern Munich face a fight for their Champions League lives this week in the penultimate round of group phase matches.
Mediocre starts by the premier league leaders and the German champions mean their tough respective trips to Inter Milan and Celtic have become games they must not lose.
Only two teams, Real Madrid and Juventus, have so far been guaranteed a place in the knockout phase.
The penultimate round is often the most confusing, with qualification hopes obscured in a fog of mini-leagues and head-to-head calculations.
But the simple fact is that only the top two teams in each of the eight groups qualify for the knockout phase.
Arsenal, despite a superb unbeaten start in the premier league, lie bottom of Group B on four points, albeit only three points behind leaders Inter.
Sandwiched between the two and still very much in contention are Ukrainian side Dynamo Kiev (6 points) and their Russian rivals Lokomotiv Moscow (5). They meet in the Russian capital.
Bayern's stuttering start in Group A -- they are third on five points but only two behind leaders Olympique Lyon -- could cost them dear if their players fail again at Celtic Park, one of Europe's most intimidating grounds.
Martin O'Neill's side have been in superb form in the Scottish premier league and victory on Tuesday would virtually assure Celtic of qualification.
DOMINANT CHELSEA
French side Monaco, astonishing 8-3 winners over Deportivo Coruna in their last game, will qualify from Group C, which they top with nine points, if they avoid defeat at home to Dutch side PSV Eindhoven.
Spain's Real Sociedad are firm favourites to win the race for second spot behind last year's losing finalists Juventus in Group D.
Sociedad travel to Greek side Olympiakos with a four-point advantage over both their opponents and the fourth team in the group, Galatasaray, who host Juve in a match postponed until December 2 due to the bomb attacks in Turkey last week.
Manchester United and VfB Stuttgart of Germany are in pole position in Group E with nine points apiece. They will qualify if they avoid defeat away to Panathinaikos and at home to Rangers respectively on Wednesday.
UEFA Cup holders Porto will join nine-times winners Real Madrid in qualifying from Group F if they beat visiting Partizan Belgrade.
Chelsea dominate Group G with nine points and will expect to clinch qualification with a flourish by beating Czech side Sparta Prague at Stamford Bridge.
Lazio and Besiktas meet in Rome in the group's other game with the Italians, two points behind their Turkish opponents, badly in need a victory.
Group H is extremely tight with reigning European champions AC Milan, top with seven points, visiting Ajax Amsterdam (6 points). Celta Vigo (5) and Belgium's Club Bruges (4) meet in Spain hoping to profit from any slip-up by the leading pair.