Goals from Gary Hooper and Victor Wanyama saw Celtic ease past Helsingborg 2-0 and into the group stage of the Champions League for the first time since the 2008-09 season.
Neil Lennon's men took a 2-0 lead into the second leg of their playoff, with Hooper opening the scoring on the half-hour mark before Wanyama sealed a 4-0 aggregate victory with a late header.
Lennon claimed he had "been to hell and back" before the "best night" of his managerial career in guiding Celtic past Helsingborg.
"There is a fear factor and I lived with that," Lennon said. "I am thinking of the ramifications if we don't go through, what it would mean for me personally, how I would get through it and how I would get over it."
French club Lille needed Tulio De Melo's extra-time goal to advance to the group stage of the Champions League on Wednesday at the expense of FC Copenhagen.
The Brazilian striker headed in a cross from Nolan Roux in the 105th minute to give Lille a 2-0 victory and a 2-1 win on aggregate. Lucas Digne scored in the first half to tie the two-legged match 1-1.
Elsewhere, Borussia Moenchengladbach beat Dynamo Kiev 2-1, but was eliminated 4-3 on aggregate, and Celtic progressed to the lucrative group stage for the first time since 2008 by beating Helsingborg 2-0 for a 4-0 overall win. Spartak Moscow drew 1-1 at Fenerbahce to go through 3-2, and Cluj ousted FC Basel, which made the last 16 of the competition last season, by winning 1-0 and 3-1 overall.
Information from Press Association and The Associated Press was used in this report.