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.The 31 papers and 1 poster paper were carefully reviewed and selected for inclusion in this book. They stem from the following tracks: 5 papers were selected from a total of 48 submissions to the Research Track: Smart Cities and Smart Mobility, CSCI-RTSC; another 5 papers were selected from 46 submissions to the Research Track: Software Engineering, CSCI-RTSE; and 22 papers were selected from 52 submissions to the Research Track: Education, CSCI-RTED.The papers were organized in topical sections as follows: Smart Cities and Smart Mobility; Software Engineering Research and Practice; Education - Mathematics; Education - Teaching and Learning Strategies, Curriculum Development, and Tools; and Poster Papers.