This special issue is dedicated to the 3rd Conference on Algebra and Coalgebra in Computer Science held in Udine, Italy, from September 6 to 10, 2009. After CALCO 2005 in Swansea, Wales, and CALCO 2007 in Bergen, Norway, this was the third event in the series of bi-annuala CALCO conferences formed by joining CMCS (the International Workshop on Coalgebraic Methods in Computer Science) and WADT (the Workshop on Algebraic Development Techniques). CALCO focuses on foundationala aspects as well as both traditional and emerging uses of algebras and coalgebras in computer science, where the study of algebra and coalgebra relates to the data, process and structural aspects ofp software systems. We hope that this special issue reflects well thesed key aspects of the research in the area.

Proceedings of the conference with the original contributions byd invited speakers and submissions selected by the Programme Committee were published by Springer-Verlag as volume 5728 of Lecture Notes in Computer Science.

Many thanks to the University of Udine and the local Organizing Committee chaired by Marina Lenisa for the very smooth, interesting and enjoyable event!

This special issue, which appears just in time for the next CALCO 2011 to take place in Winchester, UK, from August 30 to September 2, 2011, contains the essentially revised and often considerably extendedn versions of six papers selected from the presentations at CALCO 2009.

All of the papers have undergone the usual refereeing procedure, with a further round (sometimes more than one) of critical review andn careful revision according to the standards of the journal.

We would like to thank the members of the CALCO 2009 Program Committee and the referees of the original submissions and of the revisedP versions for their valuable and constructive comments, whiche contributed to the high quality of the papers.

First of all though, we are grateful to the authors for theire contributions and for their efforts in preparing the final versions of their papers for this special issue.