Within the Polish psychotherapist community, there are differences of perspective regarding, among other things, the definition of psychotherapy, its training, and the role of diagnosis in the psychotherapeutic process. These differences have been particularly pronounced during the development of the Act on the Profession of the Psychotherapist. This article seeks to demonstrate the relevance of diagnosis in the psychotherapeutic process and to outline its forms. In the field of psychotherapy, diagnosis refers to three areas and three types of assessment: medical diagnosis (identification), psychological diagnosis, and therapeutic diagnosis. The aforementioned forms of diagnosis are not mutually exclusive and can therefore be used complementarily and employed narratively throughout the process of psychotherapy. The author also presents selected definitions of psychotherapy proposed by Polish psychotherapists Jerzy Aleksandrowicz, Czesław Czabała, and Bogdan de Barbaro. Furthermore, she engages in a debate on the role of diagnosis in psychotherapy with the perspectives of prominent U.S. psychotherapists such as Irvin Yalom, Harlene Anderson, Michael White, and David Epston. In doing so, the author draws upon her many years of clinical and therapeutic practice, whilst also acknowledging the differing opinions of other experienced practitioners.