This book constitutes the proceedings of the 21st International Conference on Passive and Active Measurement, PAM 2020, which was planned to be held in Eugene, Oregon, USA, in March 2020. Due to the Corona pandemic, the conference was organized as a virtual meeting.The 19 full papers presented in this volume were carefully reviewed and selected from 65 submissions. They were organized in topical sections named: active measurement; security; best practices and conformance; domain names; topology and routing; topology - alias resolution; and Web.