This book constitutes the post proceedings of the 28th International Symposium on Modelling, Analysis, and Simulation of Computer and Telecommunication Systems, MASCOTS 2020, held online -due to COVID -19- in Nice, France, in November 2020. The 17 full papers presented were carefully reviewed and selected from 124 submissions. The symposium collected the most relevant papers describing state-of-the-art research in the areas of the performance evaluation of computer systems and networks as well as in related areas.