This article is based on a qualitative case study of teachers' conceptions of improvisation in teaching. Empirical data are master student teachers' texts (transcripts, reflections) based on observations and interviews of practising teachers. The texts were analysed in an abductive process. We can identify four specific characteristics of how improvisation in teaching is conceived; improvisation of design, improvisation in communication, and improvisation dependent on repertoire and context. However, teachers experience severe challenges in their improvisational practices, e.g. with regard to their knowledge base, the accountability agenda and teacher autonomy. Therefore, we argue that improvisation should be part of teacher education.