Experts in marketing, brands and opinion make a large use of the concept of reputation which fits quite well the need for metrics made available by digital traces, ranging from mass media to social networks. The paper proposes a precise description of the mediations that produce these metrics. Various dimensions of reputation emerge from this exploration (aggregation, action, label, confinement) and their technical and cognitive features are carefully observed to understand how they produce an assemblage designed on purpose. This mediology of reputation, i.e. the study of media formats and their specific agency, puts in light the composition of conventions which is a prerequisite for reputation to become taken for granted and real.