The aim of the study is to discuss experience and reflection referring to an empirical study of 54 primary science trainee teachers’ reflection on their science teaching to portray important factors for primary science trainee teachers´ professional development. I discuss and articulate how they reflect on why they chose a particular content for their instructions, how they interact with the pupils, the pupils’ learning, and their own learning and development of Pedagogical Content Knowledge. I use the Shulman model of Pedagogical Reasoning and Action to make a process analysis and to portray trainee teachers’ development of a new comprehension through reflection and evaluation of the process. I also use the Content Representation (CoRe) to stimulate trainee teachers’ reflection. The CoRe outlines some of the aspects of PCK `most attached to the content` e.g. knowledge of alternative conceptions, insightful ways of testing for understanding, known points of confusion, effective sequencing and important approaches to the framing of ideas. I also used questions to the pupils aimed to focus on their experiences of the lesson and also how they experience their own learning. When trainee teachers engage in reflective processes that contribute to constructing their knowledge, they begin to understand the tentativeness of knowledge about science teaching and learning.