This two-volume proceedings contain the papers presented at CSP 2012: 21th international Workshop on Concurrency, Speci cation and Programming held on September 26-28, 2012 in Berlin. The workshop has been organized since the early seventies alternatively by Warsaw University and Humboldt-University (since 1993 as CSP) . Over the time, it has been extended from a bilateral seminar to a meeting attended also by colleagues from other countries than Poland and Germany. Presently there are 73 participants from 10 countries. Following its tradition, we strive to retain an informal working atmosphere. Therefore, the call for papers welcomed drafts and extended abstracts, as well as, fully elaborated contributions.