Irish international John O'Shea's 17th minute goal was the difference between Manchester United and Arsenal after their Champions League semifinal first leg at Old Trafford.
O'Shea got on the end of a Michael Carrick cross and brilliantly half-volleyed past Almunia to give the Reds a deserved 1-0 lead.
United dominated Arsenal in the first 45 minutes and the Gunners can thank Manuel Almunia for keeping them in the game with a host of splendid saves.
The pace slackened somewhat in the second period as both sides were afraid of making a mistake that could turn the tie on its head. Ronaldo came close to a second in the 69th minute for United when his 35-yard piledriver hit the post with Almunia beaten all ends up.
Arsenal substitute Niklas Bendtner had an outstanding opportunity in the 86th minute to sneak an away goal for the visitors, getting first to a cross but fluffing the chance by heading over the bar.
United will be delighted to have scored and kept a clean sheet, but the tie is still very much alive and the Gunners know what they have to do at the Emirates Stadium next week.