This book constitutes the thoroughly refereed post-proceedings of the 12th International Workshop on Coalgebraic Methods in Computer Science, CMCS 2014, colocated with ETAPS 2014, held in Grenoble, France, in April 2014. The 10 revised full papers were carefully reviewed and selected from 20 submissions. Also included are three invited talks. The papers cover a wide range of topics in the theory, logics and applications of coalgebras.