This book constitutes the refereed proceedings of the 18th International Conference on Critical Information Infrastructures Security, CRITIS 2023, held in Helsinki region, Finland, during September 13-15 2023. The 13 full papers and 3 short papers and one Paper in Progress were carefully selected from 25 submissions. The conference addressed four scientific focus areas, namely information, infrastructures, security and hybrid threats in an interdisciplinary manner and specifically in the context-based analyses and complex optimization methods.