This chapter is about evolutionary application development as a means for including end users in further development of existing software systems. The chapter presents concepts and techniques for evolutionary development that reuses ideas from other domains in which evolution plays a dominant role (biology, architecture, and art objects). It argues that end users need access to representations of a system that are less formal than program code, but more powerful than informal design representations, and that this information can be obtained from the system’s past use and development history. The “resemblance relation” is presented as a tentative solution. It includes elements of object-oriented programming and component-based development. It is hoped that this chapter will provide the reader with a new view on systems development, and how end users can participate as designers in the evolutionary process.