Language prototyping is the activity of designing and testing definitions of new or existing computer languages. An important aspect of a language definition is the definition of its syntax. The subject of this thesis are new formalisms and techniques that support the development and prototyping of syntax definitions. There are four main subjects: (1) Techniques for parsing and disambiguation of context-free languages. (2) Design and implementation of a new syntax definition formalism. (3) Design of a multi-level algebraic specification formalism. (4) Study of polymorphic syntax definition.