This IFIP report is a collection of fundamental, high-quality contributions on the algebraic foundations of system specification. The contributions cover and survey active topics and recent advances, and address such subjects as: the role of formal specification, algebraic preliminaries, partiality, institutions, specification semantics, structuring, refinement, specification languages, term rewriting, deduction and proof systems, object specification, concurrency, and the development process. The authors are well-known experts in the field, and the book is the result of IFIP WG 1.3 in cooperation with Esprit Basic Research WG COMPASS, and provides the foundations of the algebraic specification language CASL designed in the CoFI project. For students, researchers, and system developers.