Advances in cognitive psychology and in research techniques have led to an increase in the acceptance of the conception of teaching as a “thoughtful” profession. The interest and enthusiasm of researches in aspects of teacher cognition demonstrate a shift from an emphasis on observable teacher behaviours to a focus on a teacher's unobservable thinking process. In this study, a qualitative approach was used to uncover a teacher's thinking process during lesson planning, to depict a more holistic view of the structural complexity of teacher cognition during lesson planning. Specialised science teachers and general teachers who had different levels of subject expertise were studied. The teachers were interviewed on how they planned an elementary science lesson. Interview protocols were analysed using a taxonomy which assessed the cognitive complexity of teacher thinking. Differences were found between specialised science teachers and general teachers in the levels of structural complexity in their thinking process. Copyright © Springer 1997.