Developments in packaging reflect the fact that manufacturing economies are related to the level of integration achieved on electronic assemblies. Limiting the natural trend to build large, complex, highly-integrated assemblies is the corresponding drop in production yield. In recent years, however, yields for complex assemblies both in semiconductor chips and printed circuit boards have risen substantially. The effect has been to produce electronic assemblies whose complexities have placed new emphasis and new demands on testing. Traditional approaches to the problems of test and repair have, for the most part, been abandoned in favor of more manageable, cost-effective approaches. This paper describes the techniques and test system developed by Xerox for its large digital modules, beginning with a discussion of the issues peculiar to the test and repair of such modules.