Irrespective of what approach is taken to the development of computational thinking, or at what age it is introduced, the teacher is central to ensuring that the children they work with develop computational thinking. It is therefore essential that their teachers are adequately prepared to include computational thinking as part of their pedagogical classroom practices. Moreover, it is argued that this preparation should begin at pre-service level. Adopting a constructionist perspective of learning, this paper presents and discusses the findings from research that investigated preservice teachers’ understandings of computational thinking, having completed a specialism in digital learning, the final activity of which entailed using computational tools with children in the classroom as part of a primary school science curriculum. Findings indicate that working with the children in the classroom helped the preservice teacher develop their own understandings of what computational thinking (CT) looks like “in action”, enabling them to reflect more deeply on the fundamentals of CT and on how to use CT in their own classroom practice as qualified teachers.