Assessing the impact of changes in one part of a software system on other parts remains one of the most compelling problems in software maintenance. This problem can be relieved by making implicit dependencies between different system parts explicit. We propose to explicitly document interactions between different system parts by means of reuse contracts that can only be manipulated by formal reuse operators. Reuse contracts and their operators facilitate managing the evolution of a software system by indicating how much work is needed to update the system, by pointing out when and which problems might occur and where and how to test and adjust the system.