Real Madrid are aiming for a third straight Champions League title as Europe's top club competition kicks off. Barcelona play Juventus in a key first-matchday game, while the spotlight falls on big-spending Paris Saint-Germain, whose campaign gets underway at Celtic.Here's a look at the eight groups:

GROUP A (BENFICA, MANCHESTER UNITED, BASEL, CSKA MOSCOW)

Jose Mourinho can be happy with how Manchester United's return after a one-year absence is shaping up, along with his own quest to be the first person to coach three different clubs to European glory. Portuguese Benfica are a consistent performer in the second tier of Europe's elite. Swiss side Basel, on the other hand, have been weakened by persistent sales of top players since eliminating United in the 2011 groups. Meanwhile, CSKA Moscow will look to improve their European records after finishing last in their group in each of the last four seasons, including one that contained United in 2015.

GROUP B (BAYERN MUNICH, PARIS SAINT-GERMAIN, ANDERLECHT, CELTIC)

Neymar made it his big goal to win the Champions League with Paris Saint-Germain after securing his world-record transfer from Barcelona for 222 million euros in July. Advancing to the knockout stage should be a formality for his new team, which also include teenage striker Kylian Mbappe in a new-look and exciting forward line. Neymar's first European campaign with PSG will take him to five-time champion Bayern, whose coach Carlo Ancelotti used to manage the French club. Scottish giants Celtic are never an easy team to visit, but they are likely to be fighting it out with Anderlecht for third place.

GROUP C (CHELSEA, ATLETICO MADRID, ROMA, QARABAG)

Qarabag are the first Azerbaijani team to make it to the group stages and their reward was one of the most competitive groups. Atletico Madrid have reached the final twice in the past three years, losing both times to Real Madrid. Meanwhile, Chelsea, the 2012 European champions, are the current English champions and have recovered after an uncomfortable start to the Premier League. Turkish football fans will keep their eyes on the other team in this group, Roma, to see how their Turkish wunderkind Cengiz Ünder fares on European football's biggest stage.

GROUP D (JUVENTUS, BARCELONA, OLYMPIAKOS, SPORTING)

It will be a major surprise if Juventus and Barcelona, European champions a combined seven times, fail to qualify from the group. Neymar, of course, is no longer around for Barca, with new signing Ousmane Dembele replacing him. Juventus lost last season's final to Real Madrid and haven't won the Champions League since 1996. Greek holders Olympiakos, and Sporting are regular qualifiers but rarely advance, with the latter weakened by their recent sale of midfielder Adrien Silva to Leicester.

GROUP E (SPARTAK MOSCOW, SEVILLA, LIVERPOOL, MARIBOR)

Five-time European champions Liverpool came through the playoffs and got a chance to avenge their loss to Sevilla in the 2016 Europa League final, which denied the English team a place in last season's Champions League. Spartak, which won the Russian Premier League, are in the group stages for the first time since 2012-2013, while Slovenian team Maribor will be the big outsider in their third attempt to reach the knockout stages. This will likely be considered the weakest of the eight groups.

GROUP F (SHAKHTAR DONETSK, MANCHESTER CITY, NAPOLI, FEYENOORD)

Pep Guardiola's Manchester City avoided tougher options by being drawn into top-seeded Shakhtar's group. Still, Napoli were one of the more difficult opponents for City. Feyenoord return to the group stages after a 15-year absence and are likely to face a steep learning curve.

GROUP G (MONACO, PORTO, BEŞİKTAŞ, LEIPZIG)

Last season's surprise semifinalist Monaco are the top seed but have been hurt by the departure of key players like Kylian Mbappe, Bernardo Silva and Benjamin Mendy this summer. Porto will hope to extend their record of never having lost to Turkish opposition in eight previous matches when they kick off Group G with a visit from Beşiktaş Wednesday.

Leipzig didn't even exist then; the club was created in 2009, and are newcomers at this level.

GROUP H (REAL MADRID, BORUSSIA DORTMUND, TOTTENHAM, APOEL)

Real Madrid's path to the knockout stages may have been smoothed by their fierce rival Barcelona, which weakened Borussia Dortmund by signing Ousmane Dembele. Tottenham will be hoping for better results in their temporary home at the Wembley Stadium. APOEL will be inspired by their 2012 performance when they famously reached the Champions League quarterfinals against the odds.