In this chapter, we describe and exemplify formative assessment in primary science as a cyclical process. We identify points at which the cycle can become stalled, explaining why the use of assessment strategies may not always have their desired impact. We also explore opportunities for the formative assessment cycle to be successful within primary science lessons using examples from classrooms in Switzerland and the United Kingdom (UK). From Switzerland, we draw on results from the ASSIST-ME (Assessment In Science, Technology and Mathematics Education) project which explored formative assessment practices in science inquiry classrooms (Grob et al., 2021). From the UK, we draw upon evidence from the TAPS (Teacher Assessment in Primary Science) project and the Focus4TAPS professional learning programme which have been found to have a positive impact on both children’s learning and teacher confidence in science teaching (Mujtaba et al., 2022). We argue that formative assessment is essential in primary science, to support children to move from common-sense explanations of the world to more scientific viewpoints. Use of such primary science teacher assessment capability is a developmental process that needs to be supported by ongoing professional learning opportunities and exemplification, enabling the adaptation of lessons to meet the needs of the learners.