This book constitutes the refereed post-conference proceedings of the 18th International Conference on Mobile and Ubiquitous Systems: Computing, Networking and Services, MobiQuitous 2021, which was held in November 2021. The conference was held virtually due to the COVID-19 pandemic.The 37 full papers were carefully reviewed and selected from 79 submissions and present discussions, interaction and exchange of experiences that will designate future research efforts and directions. Topics addressed by the conference include systems, applications, social networks, middleware, networking, sensing, data management, data processing and services, all with special focus on mobile and ubiquitous computing.