This book constitutes revised selected papers of the 19th International Conference on Information Technologies and Mathematical Modelling, ITMM 2020, named after A.F. Terpugov, held in Tomsk, Russia, in December 2020. The 31 full papers presented in this volume were carefully reviewed and selected from 82 submissions. The conference covers various aspects of information technologies, focusing on queueing theory, stochastic processes, Markov processes, renewal theory, network performance equation and network protocols.