The current tendency of the European educational strategy on the compulsory children’s involvement in the preschool education before elementary school raises the issue of preparing children for school and the preschool educationfunctioning, modernization and updating the content and technology of pedagogical activities at this stage. The article highlights the innovation of the Polish theory and practice in preparing a child for school i.e. the transfer of the pedagogical diagnosis function in school readiness from psychologists to preschool teachers. The organization of systematic, daily observation and interpretation of children's activities has become an integral part of the teacher's work. The essence of systematic pedagogical diagnostics of school readiness in Polish preschool institutions is revealed. On the basis of retrospective analysis, it is highlighted the evolutionary development in Polish pedagogy of the diagnosing children's readiness problem in school. According to the chronological principle, diagnostic methods are systematized; their semantic and procedural aspectsare characterized. It is substantiated that as a result of diagnostic techniques, mastering the tools and ability to perform diagnostic procedures, the teacher gets the opportunity to determine the degree of mental and social development of the child, which contributes to school tasks, as well as factors that determine this development. The teacher forms a complex child’s profileconcerningthe knowledge and competencies, and on its basis develops a strategy of pedagogical influence and interaction with the child at the beginning of school