The understanding of complex, dynamic and animate systems has a special standing in education for sustainable development and biology. Thus one important role of science teacher education is to promote student teachers' Content Related Knowledge (CRK) for teaching systems thinking, consisting of extensive Content Knowledge (CK) and well formed Pedagogical Content Knowledge (PCK). But the effective development of this knowledge is empirically uncertain. This article reports the effects of three different interventions (technical course, didactic course, mixed course) in promoting student teachers' reflective Content Related Knowledge for teaching systems thinking. Participants analyzed a videotaped lesson of 6th graders learning about the ecosystem lake and identified weak aspects in the teacher's CK and PCK. The results revealed that student teachers' reflective Content Related Knowledge for teaching systems thinking can be promoted in teacher education. The conclusion to be drawn from our findings is that a mixed course with equal technical and didactical input seems to be most effective in fostering student teachers' reflective Content Related Knowledge for teaching systems thinking.