Lisbon has a special place in the minds and hearts of Celtic supporters, for obvious reasons. It was there that the club recorded the greatest success in their history, beating Inter 2-1 to win the European Cup in 1967, becoming the first British club to do so.
Yet more recently, things haven't been quite so memorable when Celtic have had to travel to the Portuguese capital. In their past two Champions League outings away to Benfica, Celtic have lost 1-0 and 3-0 and while a defeat tonight wouldn't kill their chances of qualification, it would put them in a far from ideal position.
Here are five key aspects of the game which could prove decisive if Celtic are to continue their impressive Champions League form tonight.