Abstract: A trend in software development is to assemble a system from a number of components. These may be either available commercially off-the-shelf, or by the use of network-based resources. In many cases, the system is expected to function for multiple configurations of interchangeable components. The trade off that a system tester faces is the thoroughness of test component configuration coverage, versus limited resources of time and expense. This paper presents a metric that can be used to measure component interaction coverage of a set of system test configurations. We also provide a formal definition of the system test interaction problem.