The intent of this research is to cast the prototyping method in a formal setting and to demonstrate the feasibility of such a formalization. To grasp the nature of prototyping, as opposed to traditional methods, concepts and issues that form the essence of prototyping are identified. These are: linguistic framework, formality, and abstraction levels. A multilevel system based on three languages, each at an appropriate level of abstraction, is developed. Denotational semantics and a corresponding implementation are used to formalize the notion of transformation from one level to the next.