This book constitutes the proceedings of the 17th International Colloquium on Theoretical Aspects of Computing, ICTAC 2020, which took place during November 30-December 4, 2020. The conference was originally planned to take place in Macau, China, but changed to a virtual only format due to the COVID-19 pandemic. The 15 papers presented in this volume were carefully reviewed and selected from 40 submissions. The book also contains one invited talk in full paper length. The book deals with challenges in both theoretical aspects of computing and the exploitation of theory through methods and tools for system development.