Open Multiparty Interaction.- Behaviour Protection in Modular Rule-Based System Specifications.- Quantitative Modal Transition Systems.- Bounded Model Checking of Recursive Programs with Pointers in K.- A Probabilistic Strategy Language for Probabilistic Rewrite Theories and Its Application to Cloud Computing.- Adaptable Transition Systems.- Compiling Logics.- Transformation Systems with Incremental Negative Application Conditions.- Statistical Model Checking for Composite Actor Systems.- Barbed Semantics for Open Reactive Systems.- Designing Domain Specific Languages - A Craftsman's Approach for the Railway Domain Using Casl.- Satisfiability Calculus: The Semantic Counterpart of a Proof Calculus in General Logics.- Semantics of the Distributed Ontology Language: Institutes and Institutions.- Formal Specification of the Kademlia and the Kad Routing Tables in Maude -- A Generic Program Slicing Technique Based on Language Definitions -- Distances between Processes: A Pure Algebraic Approach.

This book constitutes the thoroughly refereed post-conference proceedings of the 21st International Workshop on Algebraic Development Techniques, WADT 2012, held in June 2012, in Salamanca, Spain. The 16 revised papers presented were carefully reviewed and selected from 25 presentations. The workshop deals with the following topics: foundations of algebraic specification; other approaches to formal specification including process calculi and models of concurrent, distributed and mobile computing; specification languages, methods, and environments; semantics of conceptual modeling methods and techniques; model-driven development; graph transformations, term rewriting and proof systems; integration of formal specification techniques; formal testing and quality assurance; validation, and verification.