This book constitutes the proceedings of the 23rd International Conference on Scientific Computing, CSC 2025, and the 21st International Conference on Foundations of Computer Science, FCS 2025. They were held as part of the 2025 World Congress in Computer Science, Computer Engineering, and Applied Computing, CSCE 2025, which took place in Las Vegas, NV, USA, during July 21-24, 2025.The CSC 2025 Conference received a total of 210 submissions, of which 47 papers were accepted. The FCS 2025 Conference received 60 submissions, of which 9 papers were accepted. This volume includes 35 of the accepted papers from CSC 2025 and FCS 2025. The papers were organized in topical sections as follows: Military and Defence Modeling and Simulation; Scientific Computing and Utilization of Artificial Intelligence; Quantum Computing, Security, and Applications; Scientific Computing, Distributed Processing, Optimization, Numerical Methods, and Applications; 21st International Conference on Foundations of Computer Science, FCS 2025; and Poster Research Papers.