This doctoral research project aims at investigating science teachers in junior secondary education and their actions and reflections concerning evaluation as didactic tools. The changing identity of the science teacher is in particular important when it comes to the implementation of a combination of formative assessment and summative assessment. The overall perspective of the fieldwork and the analysis is a situated /socio-cultural epistemological viewpoint in which assessment is seen as a part of practically every learning activity, at the planning, execution and retrospective steps. I discuss the relative importance (for the teachers) of formative and summative evaluation within three different national educational contexts, the Norwegian, the Swedish and the English. The methodological approach has been a combination of ethnography with grounded theory. The heart of the study is teaching as a learning profession emphasizing teachers as reflective practitioners. The analysis has been carried out according to a two dimensional framework; firstly epistemological theories with their different emphasis on evaluation and assessment, and secondly according to ideologies of science education that emphasize the implicit subject related positioning behind the content and the processes of teaching natural sciences. Results will be presented as reconstructed teacher-cases along these dimensions, of which three will be discussed; Alfa, Pi and Omega.