The article deals with the problem of differential diagnosis of extreme reticence or selective speech in a child, categorised as selective mutism and shyness. Selective mutism is an increasingly recognized disorder among preschool and school children. It manifests itself functionally in the sphere of speech and communication, but in relation to the anxiety factor. As an anxiety disorder, it is categorised in the latest medical classifications ICD-11 and DSM-5, and therefore, primarily psychological or psychiatric therapeutic intervention could be expected. The specificity of the pathomechanism of selective mutism, however, requires interdisciplinary activities, with a room for a speech therapist, a special pedagogue (e.g. at a public school as a supporting teacher), any other pedagogue working with the child (educator, teacher of integrated classes, subject teacher), other specialists (therapist pedagogue, physiotherapist), as well as the parents. The speech therapist may play a special role in the diagnosis of mutism in the conditions of inclusive education, as he will probably be the first specialist who will receive a child who is not speaking or very taciturn at a public school. In the article, the diagnosis of selective mutism is associated with the differential diagnosis of shyness, which may not be treated as a disorder, but only a certain personality trait, but with incompetent pedagogical support in everyday educational practice it can lead to more serious difficulties, including logophobia and mutism. The diagnosis of mutism requires specialised therapeutic measures, but with the awareness of the differences in the situations of a shy child and a child with mutism, it is worth learning some supportive strategies that are useful in both cases.