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 42 papers were selected for inclusion in this book from the following tracks: 29 papers were selected from 168 submissions to the Research Track on Health Informatics and Medical Systems, CSCI-RTHI; 13 papers out of 294 submissions to the Research Track on Large Language Models, CSCI-LLM.The papers were organized in topical sections as follows: Health Informatics, Medical Devices, Disease Diagnosis, and Applications; Health Informatics, Artificial Intelligence, and applications; Health Informatics, Data Science, Prediction Methods, Applications and Tools; Large Language Models and Applications in Health; Large Language Models and Applications in Education; and Poster Papers.