Preface These proceedings contain the papers accepted for the main program at the 12th Majorov International Conference on Software Engineering and Computer Systems (MICSECS 2020). The conference was held in Saint Petersburg, Russia, from 10-11th December 2020. The conference is named after an outstanding scientist Sergei Majorov. He was the head of the Computer Engineering Department (1962 - 1986) and vice-chancellor for research at ITMO University (from 1962 to 1977) in Leningrad. Sergei Majorov was the founder of the scientific and pedagogical school of computing systems and networks organization and created the computational center at ITMO University and the higher school’s automated information systems department. He tirelessly worked on improving all educational process forms and bringing innovations in computer science and engineering. MICSECS 2020 features a peer-reviewing process in which every paper was reviewed in a rigorous but constructive way by at least three of the program committee members, supported by EasyChair. This year, the number of applicants slightly decrease from 2019, and as a result, 109 applications were submitted. The strict reviewing policies resulted in the acceptance of 65% to the conference and only 32 research papers into this volume. Another 39 papers in an additional volume with practical and student’s works. The authors represented 24 uni- versities, research centers, commercial companies. Our participants represented 11 countries. Established researchers and industry representatives were invited for keynote presentations at MICSECS2020. We are grateful to the keynote speakers for their insightful talks. The program also included posters, position paper presentations, and student colloquium to help attendees, especially younger researchers, to discuss preliminary ideas and promising Ph.D. topics. We want to express our gratitude to various people who made MICSECS2020 possible and the Technical Program Committee members for their valuable work in evaluating the submitted papers. March 26, 2021 Aleksandr Penskoi Pavel Kustarev Dmitry Mouromtsev 2020 Copyright for this preface by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).