Supervision is a well-defined interpersonal relationship between the thesis supervisors and their students. The purpose of this study was to identify the patterns which can explain the process of expertise attainment by thesis supervisors. We aimed at developing a conceptual framework/model to explain this development based on the experience of both students and supervisors.We have conducted a qualitative grounded theory study in 20 universities of medical sciences in Iran since 2017 by using purposive, snowball sampling, and theoretical sampling and enrolled 84 participants. The data were gathered through semi-structured interviews. Based on the encoding approach of Strauss and Corbin (1998), the data underwent open, axial, and selective coding by constant comparative analysis. Then, the core variables were selected, and a model was developed.We could obtain three themes and seven related subthemes, the central variable, which explains the process of expertise as the phenomenon of concentration and makes an association among the subthemes, was interactive accountability. The key dimensions during expertise process which generated the supervisors' competence development in research supervision consisted maturation; also, seven subthemes as curious observation, evaluation of the reality, poorly structured rules, lack of time, reflection in action, reflection on action, and interactive accountability emerged which explain the process of expertise attainment by thesis supervisors.As the core variable in the expertise process, accountability must be considered in expertise development program planning and decision- making. In other words, efforts must be made to improve responsibility and responsiveness.