We present the principles of an approach model for the prototyping-oriented development process of software systems. Because massive employment of tools is necessary for such a prototyping oriented-development, we describe the kind of toolsets which are suitable for prototyping and the requirements for prototyping-tools. The overall structure of our open CASE system, called TOPOS (an acronym for TOolset for Prototyping-Oriented Software development), and the tools for user interface prototyping and architecture and component prototyping are described in detail. By openess of a system we mean that it is easily possible to exchange tools or add new tools, i.e., expanding the toolset. We give a description of the basic concepts of the user interface prototyping-tools and architecture and com-ponent prototyping tool. The user interface and interesting parts of the implementation of these tools are presented so far it is of interest for the prototyping process.