Modern incremental and iterative software engineering processes advocate to build software systems by first creating a highly simplified and abstract model of the system which is then moved by applying a series of model improvements toward implementation. Models of software systems on any level of abstraction should contain, besides structural information, a precise description of the expected system behavior. This paper formalizes relations between models of the same system on different levels of abstraction, classifies approaches for describing behavior of system operations, and investigates how these system operation descriptions can be kept synchronized with frequent changes of the system's structure.