"Coalgebraic Methods in Computer Science" is a comprehensive collection of research papers that emerged from the 12th International Workshop on this topic. The workshop took place in April 2014 in Grenoble, France, as part of ETAPS 2014. This publication includes ten revised full texts selected from twenty submitted contributions, as well as three invited talks. The topics covered span a wide range of aspects of the theory, logic, and applications of coalgebras, providing valuable insights for researchers and practitioners in the field of computer science. The careful review and selection of contributions ensure a high level of scientific quality and relevance.