Thilo Kehrer stabbed in a late winner to keep Monaco's impressive run in this season's Champions League going with victory at Bologna.
The French side, unbeaten in this year's competition, dominated the first half but were unable to get past Bologna goalkeeper Lukasz Skorupski, who kept out Breel Embolo's strike and Aleksandr Golovin's free-kick.
Bologna have not scored in any of their games in this season's Champions League but they came close to ending that when Giovanni Fabbian's stinging second-half strike was saved by Radoslaw Majecki in the Monaco goal.
But just as the game looked to be ending goalless, former West Ham and PSG defender Kehrer fired in from a corner in the final few minutes to secure the three points for Monaco.
Monaco have won three and drawn one of their four games in the league phase of this season's Champions League, while winless Bologna are 30th out of 36 teams.