The article contains a review of basic notions connected with the consequences of an increasing multiculturalism of Polish society for the professional ethics of a psychologist in a clinical context. In the first part the issues connected with a psychological diagnosis of people of different cultural background are presented. Also, a notion of a culturally sensitive diagnosis and a specificity of interpersonal communication in a multicultural environment are described. In the second part, issues regarding psychotherapy and psychological help for people of different cultural backgrounds, with a particular attention drawn to a therapy of families culturally diverse, where the rules of communicative school of the system therapy gain an additional value are described. In both parts of the article, a particular attention is given to the contemporary understanding of the notion of cultural competency in the work of psychologists-practitioners.