PAM 2004 was the 5th International Workshop on Passive and Active Measu- ment, heldinJuan-les-PinsontheFrenchRiviera, co-organizedbytheUniversity ofCambridgeandINRIA-SophiaAntipolis, with?nancialsupportfromInteland Cisco Systems. This year we received a record number of submissions (184), re?ecting the growth of the ?eld and the critical role it plays in maintaining the network infrastructure on which we all rely. From the two-page abstracts submitted, the programme committee selected 29 papers whose authors were invited to submit full papers to appear in these proceedings. Particular emphasis was placed on selecting work that we felt was fresh and exciting, so as to encourage a dynamic and interactive workshop that provided a ?rst public presentation of research that will go on to appear in other, more formal conferences and journals. The programme committee was greatly impressed with the strength and depth of submissions received, which bodes well for the future of the subject area. This workshop took place during April 19-20, 2004 in Juan-les-Pins. Located betweentheAlpsandtheMediterranean, andclosetoNice, CannesandMonaco, Juan-les-Pins is one of the most beautiful sites on the French Riviera. Juan-les- Pins is also close to Sophia Antipolis, the French telecom valley. The workshop could not have succeeded without the support of many people whom we would like thank. First, we thank the members of the programme c- mittee for donating a considerable amount of time to review the unexpectedly large number of submissions, while working to a very tight deadline.