Observing on a diagnostic basis allows a qualified view of children’s present and potentially following mathematical competencies and is therefore vital for adaptive learning support processes. Awareness of appropriate support measures is essential for the planning of learning environments, play activities, and individually adequate interaction. The paper reports on the results of a study addressing in-service kindergarten professionals’ selection of and reflection on suitable diagnostic situations and diagnostic tools. First, aspects of professionals’ observing and diagnosing competencies will be reflected, including the use of the concept of diagnosing according to recent competence models for teachers and early childhood educators. Then, different situations suitable for observing and diagnosing children’s competencies in early mathematics education will be discussed as well as different tools for recording. Hypotheses for professional development concerning diagnostic and observation skills will be presented based on reflections and perspectives of professionals attending an in-service project.