Argumentation is a fundamental discourse of science, a part of the practice of science for evaluating, refining and establishing new theories. It is therefore considered a core element of the scientific enterprise, one which can engage students in the social practices of science. Argumentation has been the emphasis of many science education studies during the last decades and general questions about argumentation and how students and adults argue have been addressed. A central question that still remains unanswered however is Why do some students engage in argumentation whilst others do not, and what is the teacher's role in this process? This chapter provides an overview of argumentation, explains the importance of argumentation in the teaching of science, and reviews studies that explore how pre- and in-service teachers approach argumentation in their teaching. Then, in the second part of the chapter, we explore some of these issues through two case studies of teachers implementing the same curriculum in their classes, and conclude with implications for research and practice.