Classroom-based formative assessment by teachers has received a good deal of renewed scholarly and policy interest. The overall aim of this article is to foreground some of the key constitutive issues in this approach to teacher assessment and to suggest possible ways of conceptualizing key epistemological and empirical questions. This discussion raises a number of research and development issues in respect of (a) the conceptual basis of investigating this kind of student-oriented contingent assessment, (b) some methodological questions concerned with classroom-based research, and (c) the need to understand teacher development and teacher change with reference to teacher assessment practice. The concept of construct- referenced assessment (Wiliam, 2001) will be used as a point of departure for the discussion on reconceptualizing and framing the investigation of teacher assessment. A discourse-based approach will be presented in relation to classroom research. The relevance of the work in teacher development and teacher change will be discussed; some of the complexities of teacher development in relation to teacher assessment will be illustrated through data from a teacher professional development program. This discussion has a second language orientation, but many of the arguments will be relevant to language assessment in general.