This book constitutes the proceedings of the 23rd International Symposium on Fundamentals of Computation Theory, FCT 2021, held in Athens, Greece, in September 2021. The 30 full papers included in this volume were carefully reviewed and selected from 94 submissions. In addition, the book contains 2 invited talks. The papers cover topics of all aspects of theoretical computer science, in particular algorithms, complexity, formal and logical methods.

This book constitutes the refereed conference proceedings of the 10th International Conference on Algorithms and Complexity, CIAC 2017, held in Athens, Greece, in May 2017. The 36 revised full papers were carefully reviewed and selected from 90 submissions and are presented together with 3 abstracts of invited talks and a paper to the 70th birthday of Stathis Zachos. The papers present original research in the theory and applications of algorithms and computational complexity.