In the context of the impact teachers have on students' learning (teachers matter), there is an increasing interest in teachers' competences, which are seen as an amalgam of professional knowledge, beliefs, motivational orientation, and self-regulation. Following Shulman, professional knowledge in turn comprises content knowledge, pedagogical knowledge and pedagogical content knowledge. In line with these categories of professional teacher knowledge, we define the ability to identify students' errors and the knowledge about typical, domain-specific students' errors as content knowledge (CK), whereas the ability to tailor learning opportunities (errors can be used as learning opportunities), to give adequate feedback and to foster learning from errors is seen as a facet of pedagogical content knowledge (PCK). Both facets are essential if teachers want to support student learning from errors. In this chapter we define central competence facets and present the state of research on teacher competence, teacher professional competence in general and professional error competence in particular. Furthermore, we describe the German system of vocational education and teacher education and give reasons why our research is situated in the domain of accounting. These general explanations are intended to serve as a basis for all following chapters.