The article reveals the importance of diagnostics in the context of the creation of STEM centers, which are an integral part of the introduction of STEM education into the educational process. The concept of "diagnostics" in the article is considered as a process of analysis and assessment of existing organizational and scientific-methodical conditions for the creation of STEM centers. The article adds scientific knowledge about the STEM center, highlights the purpose of its activity and directions of work. Some key criteria have been identified by which the state of providing organizational conditions for the creation of STEM centers can be determined, namely: strategic planning, financial support, infrastructure, human resources, partnerships, marketing and public relations, organizational culture and management efficiency. It is proposed to evaluate the scientific and methodological conditions for the creation of STEM centers according to such key criteria as: development of STEM courses and curricula, educational materials and resources, methodical support for teachers, evaluation and analysis of learning results, research work, support of scientific consultants and experts, development cooperation networks. It was found that the diagnosis allows to assess the availability of the necessary premises, equipment, personnel support and infrastructure for the creation of STEM centers. Various methods of diagnosis make it possible to assess the availability of relevant training programs, methodological materials and other components necessary for the effective operation of the STEM center. It has been determined that such diagnostic methods as questionnaires, observations, interviews and document analysis can help in providing specific recommendations on how to improve the organizational and scientific-methodical conditions for the successful creation of a STEM center. Examples of successful innovative educational projects and experiments at the all-Ukrainian level in the field of STEM, which were implemented and are being implemented now, taking into account the results of diagnostics that helped them achieve success, are given.