The book connects systems and self-organization approaches with activity theory to create a system concept of human action. A social system is understood as an integration of elementary social systems, which are phylogenetically derived and represent a system-theoretical description of activity. Within this framework, it is examined, among other things, what can be objectified in an activity or a decision, and to what extent self-organization and cooperation serve as means to manage complexity in social systems. The work provides a foundation for software development, particularly from the user's perspective, and for decision preparation, as well as for concrete analysis and modeling.