In the light of recent trends in educational assessment, language testing and the pedagogical discussion about new teacher roles, this article focuses on the skills and abilities required of a language teacher wishing to respond to these developments, with specific reference to the primary (elementary) foreign language classroom. A new concept is introduced 'diagnostic competence' defined as the ability to interpret foreign language growth in individual children. In order to arrive at this conceptualization, data from both systematic classroom observation (The Netherlands) and ethnographic studies (Germany) are used to identify and illustrate teachers' diagnostic activities and processes. On the basis of this evidence, a preliminary description of levels and features of a teacher engaged in such activities is presented.