This paper examines the interactions between teachers’ decisions, discourses and acts, and the intended students’ learning. The focus is theoretical and methodological as it attempts to exemplify theoretical perspectives in studying mathematics teaching in its complexity. It takes into account, together or separately, the overall setting: sociocultural and institutional and the epistemological point of view on mathematics and its teaching in class. For some of the authors, the study of teacher activity in relation to students’ mathematical activity, and affective and social needs has been the focus of their research for many years, using different theoretical constructs and empirical data. As for the others, their research in the same area was focused more on the presumed cognitive needs, in relation to the practices and the mathematics at stake. The article reveals that Activity Theory has been used differently by the two traditions (English and French) as a framework for analyzing and interpreting the relations and interactions between teacher and students’ mathematical activity in research studies of the authors. This article exemplifies these different ways of using AT and discusses issues the perspectives raise for interpretation and analysis.