This paper uses case study methodology to investigate the subject conceptualisation of two geography teachers at a mixed-ability comprehensive school in the UK. The research used concept-mapping and semi-structured interviews to explore the teachers' understanding of their subjects, within the case study framework. Teachers were asked to describe their understanding of the big concepts in geography, as well as explaining what they thought the subject should include. The findings of this study suggest that the two teachers understood there to be three main strands running through geography: planning, place and process. These three themes were interlinked, and appeared to relate to some extent to big concepts within geography education, as defined within the literature. This paper illustrates the apparent relationship between a teacher's understanding of geography and their professional training, academic background and personal values; this is significant as these are likely to impact on the experience students have in the geography classroom. (Contains 5 tables and 7 figures.)