The aim of the symposium is to present and discuss the notion of “critical aspects” as a key concept for developing teaching and thus students’ learning. The concept “critical aspects” is central to phenomenography and variation theory, pinpointing what aspects students need to discern in order to develop their understanding of the object of learning in focus for the teaching. In four different presentations, with a focus on different age groups and subjects, the notion of critical aspects is discussed. In the first presentation the authors discuss how critical aspects can be used by teachers in primary school as a point of departure in mathematics teaching – in tasks as well as activities. In the second presentation the authors discuss how teachers can help kindergarteners to discern critical aspects through reading a picture book. In the third presentation the author presents a study focusing on primary teachers’ understanding of “growth mindset” and how critical aspects of this concept can be used as a foundation to inform teacher education. The final presentation discusses how teachers and teacher students understand the notion of critical aspects and what needs to be focused upon in teacher training in order for teachers/teacher students to grasp this concept in a qualified way. The symposium has scientific relevance as it opens up for different perspectives of a central concept in phenomenography and variation theory. It also has educational relevance, as the notion of critical aspects can be understood as foundational for developing teaching.