In April 2004, after one year of intense debate, CMCS, the International Workshop on Coalgebraic Methods in Computer Science, and WADT, the Workshop on Al- braic Development Techniques, decided to join their forces and reputations into a new high-level biennial conference. CALCO, the Conference on Algebra and Co- gebra in Computer Science, was created to bring together researchers and practit- ners to exchange new results related to foundational aspects, and both traditional and emerging uses of algebras and coalgebras in computer science. A steering committee was put together by merging those of CMCS and WADT: Jiri Adamek, Ataru Na- gawa, Michel Bidoit, José Fiadeiro (co-chair), Hans-Peter Gumm, Bart Jacobs, Hans- Jörg Kreowski, Ugo Montanari, Larry Moss, Peter Mosses, Fernando Orejas, Fr- cesco Parisi-Presicce, John Power, Horst Reichel, Markus Roggenbach, Jan Rutten (co-chair), and Andrzej Tarlecki. CALCO 2005 was the first instance of this new conference. The interest that it generated in the scientific community suggests that it will not be the last. Indeed, it attracted as many as 62 submissions covering a wide range of topics roughly divided into two areas: Algebras and Coalgebras as Mathematical Objects: Automata and languages; categorical semantics; hybrid, probabilistic, and timed systems; inductive and co- ductive methods; modal logics; relational systems and term rewriting.