This book constitutes the refereed proceedings of the 6th International Workshop on Passive and Active Measurement, PAM 2005, held in Boston, MA, USA in March/April 2005.The 24 revised full papers and 12 revised short papers presented were carefully reviewed and selected from 84 submissions. The papers are organized in topical sections on TCP measurements, application measurements, network inference and problem diagnosis, topology measurements, wireless network measurements, monitoring facilities, routing and traffic engineering measurements, and spectroscopy and bandwidth estimation.