This book constitutes the proceedings of the 23rd International Conference on Implementation and Application of Automata, CIAA 2018, held in Charlottetown, PE, Canada, in July/August 2018. The 23 regular papers presented in this book together with 4 invited papers were carefully reviewed and selected from 39 initial submissions. The topics of the papers include state complexity of automata, implementations of automata and experiments, enhanced regular expressions, and complexity analysis.