This book constitutes the proceedings of the 12th International Computer Science Symposium in Russia, CSR 2017, held in Kazan, Russia, in June 2017. The 22 full papers presented in this volume were carefully reviewed and selected from 44 submissions. In addition the book contains 6 invited lectures. The scope of the proposed topics is quite broad and covers a wide range of areas such as: include, but are not limited to: algorithms and data structures; combinatorial optimization; constraint solving; computational complexity; cryptography; combinatorics in computer science; formal languages and automata; algorithms for concurrent and distributed systems, networks; applications of logic to computer science, e.g. proof theory, model checking and verification; formal and algorithmic aspects of bio-informatics; current challenges such as quantum computing.