The CCIS book constitutes selected papers of the 12th International Conference on Computational Science and Computational Intelligence, CSCI 2025, which took place in Las Vegas, NV, USA, during December 3–5, 2025.A total of 40 papers was carefully reviewed and selected for inclusion in this book. 19 papers stem from the Research Track on Big Data and Data Science, CSCI-RTBD, and were selected out of 98 submissions; in addition 27 papers from the Research Track on Cyber Warfare, Caber Defense, and Cyber Security, CSCI-RTCW are included and selected from 184 submissions.The papers were organized in topical sections as follows: Big Data, Data Science, and Applications; Cyber Warfare, Cyber Defense, Cyber Security, and Post-Quantum Security; The Use of Artificial Intelligence and Data Science in Cyber Security; Poster Papers.