Credibility is one of the most important factors that distinguish a ‘good teacher’. But what is credibility? When and why is a teacher credible? In a sociological perspective, this paper considers credibility not merely as a personal quality of the sender, but as a relationship that is always full of risks, subject to continuous negotiations between teachers and students during classroom interactions. On the basis of a literature review, this work indicates three ‘roots’ through which students can recognise a teacher as credible and grant him/her their trust: a) not only disciplinary expertise, but also didactic and communicative expertise; b) the ability to express values that students can appreciate in his/her work, such as seriousness, commitment, and justice; c) communication of a sense of attention and care for each student as a person, with his/her distinctive characteristics and needs. Finally, the paper proposes a distinction between ‘credibility of the role’, which indicates the amount of prestige and the social status enjoyed by the teaching profession and educational institutions in today’s society, and ‘credibility in the role’, which indicates how the teacher assumes and practises this role, making it credible or not credible in the concrete interactions in which he/she is involved, starting from classroom interactions with his/her students. This distinction is not only theoretically important, but it also indicates a way – bottom-up, from micro to macro – to counter the risk of bureaucratization to which contemporary educational systems are continuously exposed, especially the Italian one.