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.

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Invited Talks.- Abstract Scalars, Loops, and Free Traced and Strongly Compact Closed Categories.- Labels from Reductions: Towards a General Theory.- Adequacy for Algebraic Effects with State.- Contributed Papers.- Bisimilarity Is Not Finitely Based over BPA with Interrupt.- Algebra ? Coalgebra = Presheaves.- Strong Splitting Bisimulation Equivalence.- Complete Axioms for Stateless Connectors.- On the Semantics of Coinductive Types in Martin-Löf Type Theory.- Look: Simple Stochastic Relations Are Just, Well, Simple.- Modelling Fusion Calculus using HD-Automata.- An Algebraic Framework for Verifying the Correctness of Hardware with Input and Output: A Formalization in HOL.- Using Proofs by Coinduction to Find Traditional Proofs.- From T-Coalgebras to Filter Structures and Transition Systems.- Context-Free Languages via Coalgebraic Trace Semantics.- Towards a Coalgebraic Semantics of the Ambient Calculus.- The Least Fibred Lifting and the Expressivity of Coalgebraic Modal Logic.- Ultrafilter Extensions for Coalgebras.- Equational Logic of Recursive Program Schemes.- The Category Theoretic Solution of Recursive Program Schemes.- A Categorical Approach to Simulations.- Behavioral Extensions of Institutions.- Discrete Lawvere Theories.- Final Semantics for Event-Pattern Reactive Programs.- Complete Symbolic Reachability Analysis Using Back-and-Forth Narrowing.- Final Sequences and Final Coalgebras for Measurable Spaces.- Bireachability and Final Multialgebras.- Parametrized Exceptions.- Property Preserving Redesign of Specifications.