This bookconstitutes the refereed proceedings of the 12th International Conference andSchool of Network Science, NetSci-X 2016, held in Wroclaw, Poland, in January2016.The 12 fulland 6 short papers were carefully reviewed and selected from 59 submissions.The papers deal with the study of network models in domains ranging frombiology and physics to computer science, from financial markets to culturalintegration, and from social media to infectious diseases.