The problem of assessing of child’s adaptation level to the conditions of preschool educational establishments, the warning of possible problems that can appear during this period inevitably puts before the participants of the educational process the task of determining the adaptation criteria and indicators, choice of appropriate diagnostic tools, distribution of children for adaptation groups and the development and implementation of children educational support system. The article is devoted to the problem of children’s adaptation to preschool educational establishments. Some of the results of conducted observation research are highlighted in it. In particular, the adaptation criteria and indicators were defined by the author for the diagnostics of adaptation level, namely: the physiological parameters (overall health, frequency and duration of the disease, peculiarities of the appetite, motor activity, specific features of sleep process, the course and duration of daytime sleep); the relation to cognitive activity (the activity in the game practice, interest to the educational process); the peculiarities of communicative activities (verbal activity, interaction with adults and with peers, social skills); the characteristic of the emotional states (the child’s behavior at the time of separation from parents, general emotional state, motivational readiness to visit preschool educational establishments, preference of negative or positive emotions). Theoretical study of literature sources showed that the number of methods by which to study the process of adaptation of children to preschool educational institutions is rather limited. Therefore, during the diagnostics of the levels of adaptation of the children we used the following methods: a survey of educators and parents; the social emotions test developed by M. Lusher; the method of determining the mental condition of the children of V. Manova-Tomova; monitoring of the activities and behavior of children in different periods of time; conversations with children and parents. For the level of children’s adaptability the author conventionally divided them into three groups. The first group includes children who are relatively easy adapting to kindergarten. The second group is characterized by a long period of adaptation and inconsistency of their behavior to the requirements of educational environment. The third group includes the children whose adaptation process is associated with considerable difficulties.