This paper draws on findings from a research project funded by the Scottish Executive which analysed the gender balance in teaching and explored the underlying reasons for the decline in the number and proportion of men, particularly in secondary schools. As in other developed countries, such as Australia, the USA and Canada, the proportion of men entering teaching has declined fairly rapidly over a ten-year period. At a time when women are participating in paid work in greater numbers than ever before, their concentration in certain areas of work, particularly in the service sector and the 'caring' professions, is increasingly apparent. Despite the clarity of this trend, it is evident that responses from academics and policy-makers have been very different, with some policy-makers linking the declining proportion of men in teaching with the problem of boys' underachievement and a perceived 'crisis in masculinity', whilst some feminist writers have questioned these views, drawing on recent gender theory which questions the utility of the binary categories of 'man' and 'woman', instead suggesting that gender is performed and may have little to do with the body of the person who is involved in the particular performance. Sex and gender thus become decoupled, with the focus on individual actors freely choosing the version of gender they wish to practice. This line of argument suggests that the sex of the teacher is irrelevant; what really matters is the way in which they perform gender in the classroom. Work on the gender balance in teaching therefore provides an opportunity to reflect on underlying tensions in gender theorising and policy-making. The paper begins by considering tensions between modernist and post-structuralist accounts of sex and gender. Having outlined the underlying theoretical tensions, it then goes on to consider the accounts given by teachers and students of the reasons for their own choice of teaching as a career, their experiences in teaching and their views of the reasons underlying the declining proportion of men in teaching. The aim is to consider whether students and teachers believe that sex is an important variable structuring their lives, including their decision to become a teacher and their experiences of working as a teacher, or whether they regard gender as something which is chosen from a wide repertoire of options and is relatively free from the constraints of embodiment. In relation to research on the gender balance in teaching, the paper concludes by suggesting that there is a need to make use of the idea of gender as performance, whilst at the same time holding on to the foundational concepts of 'woman' and 'man'. This is necessary to monitor and understand the career paths and underlying power relations of women and men in teaching and to transform these over time.