This article reports on the findings of a PhD research project into the improvisatory nature of teacher expertise. The data are taken from a series of comparative case studies of seven experienced teachers working in secondary schools in the South West of England and who have been identified as being expert within their school setting. Constant comparative methods of analysis have been used to draw out themes from the data. This has contributed to a grounded theory that identifies the nature of teacher expertise. The findings that arise from the data are that teacher's expertise is best expressed as continually evolving practice, a process as opposed to an end state that reflects a prototype model. Teacher expertise is seen as fundamentally improvisatory through being socially constructed and that this has a positive impact on the quality of teaching. A grounded theoretical model of teacher expertise casts new light on how we understand advanced professional practice and this article explores the implications of this contribution to knowledge for school leaders, teachers, researchers and those with responsibility for the initial training and the continuing professional development of teachers.