The transition from preschool to primary school is considered by many researchers as being one of the most challenging transitions that children can experience in their educational development. In 1998, in an effort by the Swedish government to facilitate this transition, preschool class was introduced as a voluntary bridge year between preschool and primary school.  Changes in school legislation in 2018 made attendance to preschool class compulsory. Despite the introduction of preschool class in the Swedish system, a lack of knowledge and research about the policies and practices in preschool classes for successful supporting this transition has been observed. The aim of the present qualitative study was to characterize preschool, preschool class, and primary school teachers’ understandings of the transition from preschool to primary school as reflected in their teaching practices. Semi-structured interviews were conducted with preschool, preschool class and primary school teachers at one school and affiliated preschool in a town in the South of Sweden. Observations of the preschool class were also conducted. Theories of transition were applied to analyze the teachers’ understandings of the transition from preschool to primary school.  All of the participating teachers considered the change to compulsory attendance in preschool class to be beneficial for a successful transition for the children from preschool to primary school. Two key factors were identified that appeared to be consequential for how the teachers understood and worked with children’s transitions: (a) the organizational structures in place to support the teachers’ work, related to the teachers’ ability to coordinate with each other and have access to information relevant to prepare the children for the transition; and (b) the lack of a common pedagogical framework across the preschool and school.