This paper investigates teachers use of prior knowledge in whole class teaching contexts and draws on data from an ESRC-funded study. The paper explores how teachers conceptualise prior knowledge, principally as that which has been taught in school. It demonstrates strong teacher awareness of how the teaching under consideration fits with learning previously undertaken by the class, but less awareness of how the learning might build on prior learning outside school. The paper considers how teachers make connections between new learning and prior learning, and how those connections can variously support or confound children's acquisition of new knowledge and understanding.