The present dissertation features three, partly published research papers on corporate tax planning and corporate tax avoidance with a focus on German companies. It seeks to explain why tax expense differs substantially across firms and how this variation is influenced by various factors, including the recently much-discussed phenomenon called tax avoidance. This extended summary goes beyond the abstracts of these three papers and summarizes their research questions, research methodologies, and contributions. In addition, it concludes with a short synthesis of how the three papers relate to each other. However, it does not constitute an overall introduction to the research topic (for this purpose, see Section 2).