The relationships between coupling and external quality factors of object-oriented software have been studied extensively for the past few years. For example, several studies have identified clear empirical relationships between class-level coupling and the fault-proneness of the classes. A common way to quantify the coupling is through static code analysis. However, the resulting static coupling measures only capture certain underlying dimensions of coupling. Other dependencies regarding the dynamic behavior of software can only be inferred from run-time information. For example, due to inheritance and polymorphism, it is not always possible to determine the actual receiver and sender classes (i.e., the objects) from static code analysis. This paper describes how several dimensions of dynamic coupling can be calculated by tracing the flow of messages between objects at run-time. As a first evaluation of the proposed dynamic coupling measures, fairly accurate prediction models of the change proneness of classes have been developed using change data from nine maintenance releases of a large SmallTalk system. Preliminary results suggest that dynamic coupling may also be useful for developing prediction models and tools supporting change impact analysis. At present, work on developing a dynamic coupling tracer and ripple-effect prediction models for Java programs is underway.