The aim of the study text is to present a comprehensive insight into the issues of pedagogical diagnostics and its application in the practice of teachers in pre-primary and primary education. The study text deals with selected areas of pedagogical diagnostics, emphasizes the period of transition of a child from preschool education to the 1st grade of elementary school and the diagnostics of school maturity and readiness. It presents methods of pedagogical diagnostics and the possibilities of their implementation in the educational process with the intention of characterizing the diagnostic process as an active approach of the teacher to the educational process, to the pupil, to parents and colleagues. The text also introduces self-reflection of the teacher's work as an important professional competence of the teacher in the context of his professional learning.