The objective of this note is two-fold: first to discuss the need for prototyping in system design, and second to discuss the usefulness of the Demodesigner package in developing prototypes of user interfaces. Part I of the note discusses the concept of prototyping, various kinds of prototypes and the advantages and disadvantages of prototyping. Prototype is a model of a computer programme and its purpose is to serve as a communication medium between the user and the designer. A prototype should be much easier and less costly to develop then a full working system itself. A prototype can be used to demonstrate the feasibility of the logic of a proposed system, to show the format of input and output, or to demonstrate the features of user interface. The second part of the note is devoted to a discussion of the various features of the Demodesigner package. This part also contains an example dealing with development of a prototype with demodesigner.