One of the outcomes of the New Zealand early childhood curriculum, Te Whariki, is 'working theories'. Prior research on this concept has primarily utilised sociocultural theoretical underpinnings and neglected Piagetian constructivist theories. This paper explores ways the Piagetian concepts of equilibrium and disequilibrium can be invoked to support and challenge children's working theories. The paper draws on an interpretive, qualitative, practitioner inquiry study undertaken at an early childhood centre. The study identified six teaching strategies, often occurring concurrently, which were used to challenge and support the development of children's working theories through the invocation of disequilibrium. The Vygotskian concept of mediation was identified as underpinning these teaching strategies and is viewed as complementary to the Piagetian notions of equilibrium and disequilibrium. The paper argues that considering ways to invoke disequilibrium alongside existing socioculturally inspired teaching strategies adds richness to the existing understandings of fostering children's working theories. (author abstract)