This book constitutes the refereed proceedings of the 12th International Conference on Economics of Grids, Clouds, Systems, and Services, GECON 2015, held in Cluj-Napoca, Romania, in September 2015. The 11 revised full papers and 10 paper-in-progress presented were carefully reviewed and selected from 38 submissions. The presentation sessions that have been set up are: resource allocation, service selection in clouds, energy conservation and smart grids, applications: tools and protocols, community networks and legal and socio-economic aspects.