This book constitutes the proceedings of the 12th International Conference on Internet and Distributed Systems held in Naples, Italy, in October 2019. The 47 revised full papers presented were carefully reviewed and selected from 145 submissions. This conference desires to look for inspiration in diverse areas (e.g. infrastructure & system design, software development, big data, control theory, artificial intelligence, IoT, self-adaptation, emerging models, paradigms, applications and technologies related to Internet-based distributed systems) to develop new ways to design and manage such complex and adaptive computation resources.