This book constitutes the refereed proceedings of the 12th IFIP/IEEE International Conference on Management of Multimedia and Mobile Networks and Services, MMNS 2009, held in Venice, Italy, in October 2009, as part of the 5th International Week on Management of Networks and Services, Manweek 2009. The 13 revised full papers presented together with 5 poster papers were carefully reviewed and selected from 37 submissions. The papers are organized in topical sections on multimedia networks and systems management, multimedia quality, VoIP and vocal applications, and peer-to-peer multimedia networks.