Software systems, like many other systems, need to change in order to stay successful on the market. As has been described by Lehman’s Laws of Software Evolution, these changes cause software systems to become larger in size and more complex. As a consequence, more resources are needed to maintain, or in general, evolve a software system. Evolving software systems is therefore mastering change and system complexity. In this presentation, I show how the various data recorded for software projects can be used to identify critical parts of a software system and to help steering the evolution of software systems. Furthermore, I show how visualization techniques can be used to help software engineers to comprehend the implementation of large, complex software systems including large spreadsheets.