Lesson observations involving judgements of teaching quality are a regular feature of classroom life. Such observations and judgements are made by senior and middle managers in schools and also, very significantly, by Ofsted inspectors as a major component of their judgement on the quality of teaching in a school. Using the example of Ofsted inspection, but with arguments that can apply also to routine classroom observation by school managers, this article teases out what can reasonably be said about teaching quality based on observation. It reveals the importance, but also the limitations, of classroom observation. It stresses the tentative, context-specific nature of judgements and the need for observers to have relevant experience and insight into the complexities and imponderables of classroom observation that focuses on teaching quality. It does not deal with the different but allied issue of whether lessons themselves should be graded.