This book constitutes the refereed proceedings of the 12th International Andrei P. Ershov Informatics Conference, PSI 2019, held in Novosibirsk, Russia, in July 2019.The 18 full papers and 3 short papers presented in this volume were carefully reviewed and selected from 70 submissions. The papers cover various topics related to the Mathematics of Computing, Information Systems, Formal Languages, dependable and fault-tolerant Systems and Network, Automata Theory,  and much more. weiterlesen