This article presents a didactic framework on assessment, designed to think and analyze "assessment facts" (Chevallard, 1986) by combining scientific knowledge in assessment (in the diversity of the scientific fields concerned) and didactic knowledge. It takes into account both disciplinary content and professional realities, to define a new scientific approach to assessment, which is more didactic and deliberately anchored in the reality of classroom practices. This is illustrated in the paper with the analysis of one teacher's assessment practices in school mathematics. The notion of assessment episode is central to understand assessment in mathematics in all its forms, beyond its usual functions (diagnostic, formative, certifying), as well as assessment logic, which allows one to consider the individual dimension of teachers' assessment practices.