This book constitutes the proceedings of the 19th International Conference on Passive and Active Measurement, PAM 2018, held in Berlin, Germany, in March 2018.The 20 full papers presented in this volume were carefully reviewed and selected from 50 submissions. The papers demonstrate the import and extent to which measurements pervade systems — from protocols to performance to security. They are organized in the following topical sections: models and inference; security and privacy; CDNs; DNS; certificates; interdomain routing; and analyzing protocols.