This chapter introduces the most important interdisciplinary foundations and tools of business ethics. First, the authors discuss the philosophical foundations and concepts. The most important normative ethical theories, the notion of a reflective equilibrium, and several tools for justifying norms under dissent are introduced. Second, economic and socio-scientific foundations and tools are discussed, scrutinizing different concepts of rationality and assessments of social conditions. The importance of dilemma structures lies at the core of this section. Third, the authors present psychological foundations and tools, introducing the behavioral approach to ethics, dual process theory, and Haidt’s social intuitionist model to moral judgment, with its emphasis on the limits of reason for moral judgment. Moreover, the relevance of bounded ethicality in individuals and organizations for business ethics is outlined.