The study of a school organization, due to its complexity and multidimensionality, should be approached from a qualitative, constructivist-interpretivist perspective, in which the field diary is an important source of data and an analytical and reflective tool. In order to understand the importance of a field diary written during the study of a school organization in understanding its multiple logics of action on stage and behind the scenes, we applied the coding processes of the Grounded Theory. We concluded that such a tool has three fundamental dimensions: descriptive, argumentative and interpretive, in a dynamic that allows us to systematise, question and improve processes that are usually hidden in the deep structure of research.