This chapter presents a novel approach for conceptualising and organising computer applications for the purpose of simplifying end-user tailorability and evolutionary application development. The chapter combines ideas from software systems, end-user programming, Gestalt psychology, and everyday tools. An application unit is the basic unit of tailorability. It is defined as a visual component extended with aspects. Aspects provide user-oriented entry points into an application unit at different levels of abstraction. Tools and techniques for locating, viewing and tailoring application unit aspects are presented, and the results of a user study are reported. A goal of writing this chapter has been to present a comprehensive view of some basic technology for user participation in evolutionary application development.