This paper summarizes the initial experience with the programming language Concurrent Pascal in the design of three model operating systems. A Concurrent Pascal program consists of modules called processes, monitors, and classes. The compiler checks that the data structures of each module are accessed only by the operations defined in the module. The author emphasizes that the creative aspect of program construction is the initial selection of modules and the connection of them into hierarchical structures. By comparison the detailed implementation of each module is straightforward. The most important result is that it is possible to build a concurrent program of one thousand lines out of one-page modules that can be comprehended at a glance.