The transition of a child from preschool to primary school is one of the most important events in a child's life, but also in the life of his parents. Entering a new educational environment, the child should adapt to the expectations of the environment, interact with new friends from class and school, with teachers and other adults from the wider environment. These changes, adaptation to the new environment, and the quality of interaction that the child achieves can affect his self-esteem, self-perception, the quality of developing future relationships with the environment, and further academic success. Based on different theories, a child’s transition process can be viewed from three different aspects of the relationship among transition participants: the nature of the relationship, its characteristics, and the context in which it develops. In addition, it is important to point out that the relationships that a child will build with peers and adults in the first grade of primary school, in a new educational environment, will affect the course of building future relationships during growth and development. When analysing the quality of relationships that a child builds during the transition to a new educational environment, it is necessary to take into account the child's developmental characteristics, family characteristics, characteristics of the educational institution from which he comes and the one which he enters, interaction with peers which he formed in preschool, the relationship he has developed with educators, with peers in the new environment, and the development of relationships with teachers and members of the wider environment. Also, it is necessary to emphasize that the quality of relationship development needs to be monitored because the relationships that are being built and developed change over time. Thus, changes in the development of relationships and their stability should be observed through the time aspect within which changes occur. In addition to mutual cooperation and building relationships between all participants in the transition process, continuity in upbringing and education is essential accordingly with the welfare of the individual child. Therefore, in order to enable the child to easily adapt to the new environment and encourage the welfare of each child, mutual cooperation of all involved participants is required, especially the involvement of parents in the transition process, as shown by research results. Based on previous theoretical starting points on the importance of the process of transition from preschool to primary school for child development, building relationships with the environment, and further academic success, this doctoral dissertation sought to determine the opinion of participants in the transition process (educators, teachers, parents, professionals, and principals) in the Republic of Croatia on the existing educational transition practice, on the need for changes in its development in order to facilitate the child's transition from preschool to primary school. Since no instrument was found that would fully meet the requirements of this research, a new instrument was constructed based on the key factors of the process of transition of a child from preschool to primary school. The research was conducted using quantitative and qualitative methods. The main research involved 1288 participants included in the process of a child’s transition from preschool to primary school: 103 educators, 50 teachers, 66 professional associates, 24 principals, and 1,045 parents. The research was conducted in Brod-Posavina, Međimurje, Zagreb, Primorje-Gorski Kotar, Zadar, and Split-Dalmatia counties. The research showed that the process of a child's transition from preschool to primary school is based on a normative assessment of the child's abilities regardless of the chronological age of the child and the criteria established by Croatian law for children enrolling in primary school. The law stipulates the obligation to attend preschool programs, which gives all children equal starting educational opportunities. In Croatian educational practice, there is a procedure for testing children's skills in accordance with the developmental status of a child aged six or seven. The results of the research showed that there is no cooperation between the systems or cooperation with the parents of the preschool child, which would enable an easier transition of the child from preschool to primary school. Professional staff and principals of preschool institutions, as well as parents of preschool and first-grade primary school children, expressed dissatisfaction with the current transitional educational practice, which is especially evident from interviews with research participants. In addition, it is necessary to emphasize the difference in the expectations of the professional staff of the kindergarten and primary school in relation to the individual child, his abilities, and capabilities. By processing the results of the research, it was noticed that professional staff and principals are not familiar with the curriculum and program of preschool, i.e. the first grade of primary school, which indicates one of the reasons for discontinuity in the upbringing and education of children. Based on the theoretical assumptions of the process of transition of a child from preschool to primary school and the results obtained by this research, it was determined that the existing transitional educational practice in the Republic of Croatia is not in line with the welfare of the individual child. Therefore, the implications are given for the improvement of its quality.