Music therapy research frequently draws on the method repertoire of related disciplines. Of particular interest for research however are phenomena of the music therapy interaction and expression behaviour of the patient which are shown inadequately (or not at all) by existing instruments of collection. The development of research instruments specific to music therapy would be of interest. Among others, there is the Music Therapy Expression and Communication Scale (MAKS), a rating instrument for assessing the expression and communication behaviour of the client in the music therapy setting. The scale was developed on the basis of practical experience and evaluated in two studies with child and adolescent psychiatry clients. The scale has good psychometric properties and a high level of clinical relevance in agreement with established questionnaires. The article introduces a critical discussion of the development of the scale, the results of its evaluation and aspects of its practical application.