This text constitutes the thoroughly refereed post-workshop proceedings of the 14th International Workshop on Algebraic Development Techniques, WADT'99, held in Toulouse, France in September 1999. The 23 revised full papers presented together with three invited papers were carefully reviewed and selected from 69 workshop presentations. The papers address the following topics: algebraic specification and other specification formalisms; test and validation; concurrent processes applications; logic and validation; combining formalisms; subsorts and partiality; structuring; rewriting; co-algebras...