* Milan and Barcelona in Group H along with Ajax and Celtic
* Real Madrid face mouthwatering matches against Juventus
* Holders Bayern meet Man City, CSKA Moscow and Viktoria Plzen
* Dortmund in tough group with Napoli, Marseille and Arsenal
Seven-times European title holders AC Milan, knocked out of the Champions League by Barcelona in the last two seasons, were drawn with the Catalans in the group stage of this season's competition on Friday.
Nine-times champions Real Madrid will meet Juventus in two mouthwatering clashes in Group B which also features Galatasaray and FC Copenhagen.
Holders Bayern Munich landed CSKA Moscow, Manchester City and Czech champions Viktoria Plzen in Group D, giving them a seemingly straightforward passage to the knockout stage.
Chelsea, champions two seasons ago, face Schalke 04, FC Basel and Steaua Bucharest in Group E, having beaten the latter pair on their way to winning the Europa League last season.
Three-times European Cup winners Manchester United meet Ukraine's Brazilian-inspired champions Shakhtar Donetsk along with Bayer Leverkusen and Real Sociedad in Group A.
AC Milan and Barcelona have met six times in the last two seasons and the Catalans have come out on top.
The Serie A side lost 4-2 on aggregate in the round of 16 last year and 3-1 on aggregate in the quarter-finals two seasons ago, when the two sides also met in the group stage.
The pair were drawn in Group H, alongside four-times winners Ajax Amsterdam and 1967 champions Celtic, who faced Barcelona in the group stage last season and memorably beat them at home.
"It is a well-known group, with prestigious stadia, historical ones, with teams who have already won the Champions League," AC Milan vice-president Umberto Gandini told reporters.
Antonio Conte, who played under Carlo Ancelotti at Juventus for two years and now coaches the Serie A champions, will face his former manager when his side play Real Madrid.
"It's a tough group with fascinating matches. Real are among the favourites for the title and... Galatasaray are a team to be feared," Conte told Italian television.
"Unlike last season, we know what awaits us and the difficulty we will meet when we face the best teams in the world."
Real beat Galatasaray in the knockout stage last season.
Bayern are clear favourites to progress from their group but the treble winners are taking nothing for granted.
"We have to go into the competition with a lot of respect and a lot of focus from the very start," said Bayern CEO Karl-Heinz Rummenigge.
"We have to be especially careful not to underestimate CSKA Moscow and Manchester City. This will not be an automatic qualification but our aim is to make the knockout stage."
Group F was arguably the toughest, pitting last year's runners-up Borussia Dortmund against Napoli, Olympique Marseille and Arsenal.
"This is a brilliant, balanced group with four teams who have what it takes to survive the group stage," said Dortmund coach Juergen Klopp.
"The fact is we have to get something to put right (past results) against Arsenal and we have something to put right against Marseille," added Klopp, whose side finished bottom of a group featuring both teams two seasons ago.
"I have not been to Napoli but I am already looking forward to the atmosphere there."
Austria Vienna, the only debutants in the draw, were placed in Group G along with former Champions League winners Porto as well as Atletico Madrid and Zenit St Petersburg.
Big-spending Paris St Germain drew Olympiakos Piraeus, from austerity-hit Greece, and Anderlecht in Group C in what should be a fairly easy stroll for the French side.