There is general consensus regarding the urgent and pressing need to develop school students' computational thinking abilities, and to help school teachers develop computational thinking pedagogies. One possible reason that teachers (and students) may struggle with computational thinking processes is because they have poorly developed mental models of how computers work, i.e., they have inadequate “notional machines”. Based on a pilot survey of 44 pre-service teachers this paper explores (mis)conceptions of computational thinking, and proposes a research agenda for investigating the use of notional machine activities as a way of developing preservice teacher computational thinking pedagogical capabilities.