The purpose of this study is to examine the effects of a Swedish nation-wide professional development on participating teachers’ teaching practice and their knowledge, where collegial work, as a central part of the professional development, is taken into account as a factor to influence the outcome. We conducted statistical analyses (linear mixed models) on observational and interview data from a random selection of 105 teachers (grades 1–10/primary and secondary school). The results show effects on several aspects of teacher knowledge and teacher practice. However, the results show no connections between how the teachers have worked collegially and the observed changes in the teacher's knowledge and practice.