- Software development methodologies present two major schools of thought: one is that of structured system development and the other is that of prototyping . Between these extremes, there are many mixed approaches. But any form of prototyping, though attractive, is difficult to implement for large systems because of its implied lack of structure. This paper proposes a framework for the integration of prototyping into structured system development. We first carry out structured analysis and design; these products are then used to support systematic prototyping. The resulting prototype serves to provide the user with an iterative process of understanding and improving the requirements until they are satisfactory. This methodology for structured prototyping has four elements: interface, data, process, and system prototypes. The methodology is based on a new unified approach to the analysis and design phases.