This book presents the combined proceedings of the 15th International Conference on Computer Science and its Applications (CSA 2023) and the 17th KIPS International Conference on Ubiquitous Information Technologies and Applications (CUTE 2023), both held in Nha Trang, Vietnam, December 18-20, 2023. The aim of these two meetings was to promote discussion and interaction among academics, researchers, and professionals in the field of ubiquitous computing technologies and computer science and its applications. These proceedings reflect the state of the art in the development of computational methods, involving theory, algorithms, numerical simulation, error and uncertainty analysis and novel applications of new processing techniques in engineering, science, and other disciplines related to ubiquitous computing.