n Information hiding principle:  A  module should be able to obtain information from another module only through a well-defined interface between them. n Continuity principle:  A small change in the specifications should effect only a few modules in the system, and not the entire structure of the system. u Structuring a system around data rather than around functionality measurably enhances continuity. n Open-closed principle: The ability to modify or extend a module without affecting other modules. u A module that still can be modified is said to be open. u A module that is available for use by other modules is said to be closed.