This book constitutes the proceedings of the 12th International Conference on Green, Pervasive, and Cloud Computing, GPC 2017, held in Cetara, Italy, in May 2017 and the following colocated workshops: First International Workshop on Digital Knowledge Ecosystems 2017; and First Workshop on Cloud Security Modeling, Monitoring and Management, CS3M 2017. The 58 full papers included in this volume were carefully reviewed and selected from 169 initial submissions. They deal with cryptography, security and biometric techniques; advances network services, algorithms and optimization; mobile and pervasive computing; cybersecurity; parallel and distributed computing; ontologies and smart applications; and healthcare support systems.