¿The ten-volume set LNCS 12949 - 12958 constitutes the proceedings of the 21st International Conference on Computational Science and Its Applications, ICCSA 2021, which was held in Cagliari, Italy, during September 13 - 16, 2021. The event was organized in a hybrid mode due to the Covid-19 pandemic.The 466 full and 18 short papers presented in these books were carefully reviewed and selected from 1588 submissions. Part X of the set includes the proceedings of the following workshops: ¿International Workshop on Smart and Sustainable Island Communities (SSIC 2021), International Workshop on Science, Technologies and Policies to Innovate Spatial Planning (STP4P 2021), International Workshop on Sustainable Urban Energy Systems (SUREN-SYS 2021), International Workshop on Ports of the future - smartness and sustainability (SmartPorts 2021), International Workshop on Smart Tourism (SmartTourism 2021), International Workshop on Space Syntax for Cities in Theory and Practice (Syntax_City 2021), International Workshop on Theoretical and Computational Chemistryand its Applications (TCCMA 2021), International Workshop on Urban Form Studies (UForm 2021), International Workshop on Urban Space Accessibility and Safety (USAS2021), International Workshop on Virtual and Augmented Reality and Ap-plcations (VRA 2021), International Workshop on Advanced and Computational Methods for Earth Science applications (WACM4ES 2021).