Chelsea's chances of successfully defending their Champions League crown will be given a stiff early examination when they take on Italian champions Juventus at Stamford Bridge today.
Roberto Di Matteo's men will host the Serie A title-holders four months to the day after winning the trophy in a dramatic penalty shootout against Bayern Munich in the Allianz Arena.
They would appear to be going into the group E game in good form. But Di Matteo knows the rest of the continent will up their game every time they play the defending champions.
"Every opposition is going to play with extra motivation against the holders of the Champions League. So it makes our lives more difficult," said the Italian. "I'm aware no team have defended the trophy and I'm not surprised because it's such a difficult competition to win.
"To win it twice in a row has proved impossible so far because it's so competitive."
Juve are making their first appearance in the competition in three years, but they have not lost a Serie A game since the start of last season.
Chelsea, who could manage only a goalless draw at local rivals Queens Park Rangers in the Premier League on Saturday, should be able to call on the services of John Terry despite the England defender suffering a knee injury at Loftus Road.
In addition, Juan Mata trained at the weekend and was hoping to return to full fitness in time.
Twice European champions in the last four seasons, Barcelona start their mission to reclaim the trophy when they host Spartak Moscow at the Camp Nou in their opening group G game. After a summer of change, with the departure of coach Pep Guardiola and the promotion of his assistant Tito Vilanova, there was plenty of debate about whether they could rise to the challenge set by league champions Real Madrid, as well as on the European stage.
However, apart from the disappointment of losing the domestic Super Cup to Jose Mourinho's men, Barca have made a blistering start in La Liga, with Saturday's 4-1 victory away to Getafe their fourth win in four league games.
As a result, they find themselves eight points clear of Madrid already, a margin that was previously unthinkable this early in a season. "It is surprising that we have built up such a lead already at the top of the table but nothing more than that," said midfielder Sergio Busquets.
"There is still a long way to go. We are not thinking about other sides at the moment and concentrating on our next match in the Champions League."
While Cristiano Ronaldo has failed to find his feet, and Madrid have faltered, his rival Lionel Messi has started with all guns blazing and has already beaten his own record of goals in a calendar year with 61 so far, including six this term.
Barcelona still have their problems and have not solved the defensive weaknesses which cost them at times last season.
But perhaps the best tribute that can be paid to Vilanova is that it has been virtually a seamless transition since he took over the reins from Guardiola, who won 14 trophies in four years.
The bad news to come out of the Getafe game was an injury to veteran defender Carles Puyol, who will be out of action for over a month with a knee ligament injury.
Spartak are now coached by Unai Emery, a Spaniard who is renowned for the emphasis he puts on organisation and discipline. He knows Barcelona well too, having coached Almeria and Valencia before opting to move to Russia in the summer.
"I am happy to be up against Barcelona because they are one of the three best teams in the world, if not the best, but it is always difficult against them," said Emery. "There are more positives to playing Barca than negatives. We must go out and enjoy ourselves."
In the other game in group G, Celtic make their return to the competition after a four-year absence with a home clash against Benfica.
Despite a relative lack of experience in their squad, and some questionable domestic form of late, goalkeeper Fraser Forster believes the Scottish champions have reason to be optimistic.
"The quality of the players we face will be a big step up, but we just have to go out and do our best and hopefully that will help the squad develop and move forward," he said.
Last season's runners-up Bayern begin their latest bid for a fifth European Cup with a home clash against Valencia in Germany. It's a game that skipper Philipp Lahm will not be taking lightly.
"Wednesday will be a proper test," he said. "Valencia are a top team, they're almost always in the Champions League, and almost always one of the best three clubs in Spain."
Bayern and Valencia are in group F along with Lille and Bate Borisov, who clash in France.