Research relating to the development of computational thinking (CT) at primary school level is still in its infancy despite indications that it is most effective when introduced in primary/early secondary education. Teachers are pivotal to ensuring children develop CT, so it is essential they are effectively prepared, starting at preservice level, to incorporate CT into pedagogical practices. Grounded in constructionist principles and adopting the stance that CT be developed as part of subject areas other than computer science, this qualitative study presents findings focused on the understandings of CT of 51 preservice teachers who engaged in a digital learning specialism. Carried out over a 2-year period, the research investigated preservice teachers' understandings of CT and how the design of the specialism helped to develop their understanding. Findings highlight deep understandings of CT among the preservice teachers who were able to make connections between constructionism and the development of CT. They also demonstrated a high level of pedagogical knowledge, indicating they understood why they designed particular challenges for children as part of their classroom experience. Findings are relevant for the design of teacher preparation programmes illustrating how CT can be effectively embedded to combine theory and practice.