This paper focuses on what and how primary science student teachers and their mentors learn from planning and reflecting together on each other's science lessons for pupils aged 7-9. The student teachers had had training in scientific knowledge, but only brief experience of teaching. The mentors were well experienced in the pedagogy of teaching and mentoring, but did not feel confident about their science content knowledge and the teaching of science. Throughout the process of teaching and reflecting together the student teachers and the mentors expressed several specific examples of their joint learning. (Contains 1 table.)