The aim of this pilot study was to develop, practice, and evaluate learning environments that are supposed to foster early mathematical reasoning in kindergarten. The participating kindergarten teachers attended a multi-part PD program. The research focused on the development of their teaching skills, and their reasoning practice over one school year. The study to be presented concerns the teachers' development regarding their self-reported practice and pedagogical content knowledge, and their reasoning practice in kindergarten. First results show a positive picture in terms of the self-reported teachers' professional development and their practice of mathematical reasoning in kindergarten and indicate that is possible to establish a rich and multifaceted reasoning practice in early mathematics education already in kindergarten.