Gunners boss Arsene Wenger had talked up his young side as having all the qualities needed to progress against the odds, and so it proved as Milan were out battled for most of the encounter.
A famous victory, 2-0 on aggregate, was delivered when Spain international Cesc Fabregas smashed in a brilliant 25-yard strike with just six minutes left and Emmanuel Adebayor added a second in stoppage-time.
It was no more than Arsenal deserved, becoming the first English team to beat Milan here, having dominated for long spells and with Fabregas earlier having hit the crossbar.