This chapter represents a step towards the establishment of a new system development approach, called Cooperative Experimental System Development (CESD). CESD seeks to overcome a number of limitations in existing approaches: specification oriented methods usually assume that system design can be based solely on observation and detached reflection; prototyping methods often have a narrow focus on the technical construction of various kinds of prototypes; Participatory Design techniques—including the Scandinavian Cooperative Design (CD) approaches—seldom go beyond the early analysis/design activities of development projects. In contrast, the CESD approach is characterized by its focus on: active user involvement throughout the entire development process; prototyping experiments closely coupled to work-situations and use-scenarios; transforming results from early cooperative analysis/design to targeted object oriented design, specification, and realisation; and design for tailorability. The emerging CESD approach is based on several years of experience in applying cooperative analysis and design techniques in projects developing general, tailorable software products. The CESD approach is, however, not limited to this development context, it may be applied for in-house or contract development as well. In system development, particularly in cooperative and experimental system development, we argue that it is necessary to analytically separate the abstract concerns, e.g. analysis, design, and realisation from concrete activities and techniques. Thus we introduce a CESD model which provides a framework for handling this separation and at the same time makes it possible to identify and discuss the rich variety of relationships among concrete activities and the main concerns.