This book constitutes the refereed proceedings of the 12th IFIP WG 5.11 International Symposium on Environmental Software Systems, ISESS 2017, held in Zadar, Croatia, in May 2017. The 35 revised full papers presented together with 4 keynote lectures were carefully reviewed and selected from 46 submissions. The papers deal with environmental challenges and try to provide solutions using forward-looking and leading-edge IT technology.  They are organized in the following topical sections: air and climate; water and hydrosphere; health and biosphere; risk and disaster management; information systems; and modelling, visualization and decision support.