This thesis explores geography teachers' subject knowledge, contributing a detailed empirical study to the geography education research knowledge debate. School geography has been criticised for holding an out-dated, objectivist view of knowledge, which is widely attributed to its disconnection from academic geography. Conceptualising school geography using Bernstein's (2000) pedagogic device, this thesis applies a distinction between the ORF (official recontextualising field) and the PRF (pedagogic recontextualising field), or between curriculum as intention and reality (Stenhouse, 1975). Existing critiques are reframed, and teachers' role as recontextualisers of knowledge is examined through an ethnographic study of three secondary school geography departments. A model of degrees of recontextualisation is presented, describing how teachers transform knowledge into the content of school geography lessons. Existing accounts of teachers' conceptions of geography are developed through analysis of knowledge structures, knower structures, and the legitimation of knowledge claims. The role of perception, deductive reason, and testimony in the legitimation of knowledge claims is considered. I argue that these teachers describe geography as primarily based on empirical evidence: knowledge legitimated through perception ('actually seeing'). Deductive reason is used frequently by teachers, and tensions between deductive reason and perception are explored. Testimony was found to rarely be acknowledged as a source of knowledge, however, data generated in these departments suggest that it plays an significant role. Problematic aspects of testimonial knowledge are critically explored, and particular attention is given to the multiple, powerful roles played by chief examiners in the construction of school geography. I suggest that existing arrangements between examination boards and school subjects, and the importance given to accreditation may limit the extent to which students are given access to powerful knowledge. Differences found between departments make similarities striking, including: a common, high use of Google searches; uses of Virtual Shared Areas (VSAs); and differentiation between control of the curriculum at different Key Stages (KS), with the PRF dominant at KS3 (most departments are argued to largely ignore the National Curriculum, or reduce it to an auditing role), and the ORF dominant at KS4/5 (through the heavily regulative function of examination boards). An argument is made for developing more geographical research on geography education, through a model of teachers participating in such geographical research in order to develop their disciplined judgement.