The International Research and Practice Conference “Information Technologies and Security” is an academic event, annually held by the Institute for Information Recording of the National Academy of Sciences of Ukraine. Although the Conference is headquartered in Ukraine, its geographical scope is expanding with every new edition of the event. Participants (editors, organizing committee members, and paper authors) come from around the globe. Paper publication standards and requirements to authors are growing higher, while review process is becoming more rigorous with every coming year. The collection includes selected papers, presented at the XXI "Information Technologies and Security" Conference (ITS-2021, December 9, 2021, Kyiv, Ukraine). Out of approximately 50 initial submissions 23 best papers have been selected for publication in the proceedings volume. Selected papers cover the latest advances in such areas of Computer Science as  Big Data Technologies;  Information and Cybernetic Security;  Security and Survivability of Critical Infrastructures;  Modelling and Analysis of Computer-mediated Communication Processes;  Analytical Systems Based on Open Source Intelligence (OSINT);  Complex Systems Modelling;  Decision Support Systems. Hopefully, the present proceedings volume will provide a valuable contribution to fundamental and applied knowledge in the field of Computer Science and the listed areas.