Before re-engineering a large and complex software system, it is wise to study its change history in order to identify the most valuable and problematic parts. Unfortunately, typical change histories contain thousands of entries, therefore the challenge is to discover those changes which are relevant for both the current and future situations of our product and process. We demonstrate how a simple visualization allows us to recognize relevant changes. Applying the technique on the change history of Tomcat, we have been able to identify (a) unstable components, (b) coherent entities, (c) design and architectural evolution, and (d) fluctuations in team productivity.