This book constitutes revised selected papers of the 20th International Conference on Information Technologies and Mathematical Modelling, ITMM 2021, named after A.F. Terpugov, held in Tomsk, Russia, in December 2021. Due to the COVID-19 pandemic the conference was held in a virtual mode. The 28 full papers presented in this volume were carefully reviewed and selected from 89 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.