This book constitutes the proceedings of the 12th International Conference on Computational Methods in Systems Biology, CMSB 2014, held in Manchester, UK, in November 2014. The 16 regular papers presented together with 6 poster papers were carefully reviewed and selected from 31 regular and 18 poster submissions. The papers are organized in topical sections on formalisms for modeling biological processes, model inference from experimental data, frameworks for model verification, validation, and analysis of biological systems, models and their biological applications, computational approaches for synthetic biology, and flash posters.