Today, in general, software is embedded, distributed onto networks and structured into logical components that interact asynchronously. We study fundamental models of composed software systems and their properties, identify and describe various basic views, and show how they are related. We concentrate on models of composed systems that interact by message exchange. We consider, in particular, models of data, states, interfaces, hierarchical composed systems, and processes. We study relationships by abstraction and refinement as well as forms of composition and modularity. In particular, we introduce a comprehensive mathematical model for composed systems, its views and their relationships.