Background/Context: School leaders, teachers, and researchers are increasingly involved in collaborative research and development (R&D) projects in schools, which encourage crossing boundaries between the fields of school and research. It is not clear, however, what and how professionals in these projects learn through cross-professional collaboration. Purpose: The purpose of our study is to create a better understanding of the learning of boundary crossers who are involved in cross-professional collaboration in R&D projects. Research Design: In this multicase study, we analyzed data from interviews with school leaders, teachers, and researchers involved in 19 R&D projects in Dutch secondary schools. We interpreted boundary crossers' learning in terms of learning mechanisms (identification, reflection, coordination, and transformation) and related these learning mechanisms to different types of cross-professional collaboration. Findings: Three combinations of learning mechanisms were prevalent: identification and coordination, reflection and transformation, and transformation for school leaders, teachers, and researchers. Different types of collaboration appeared to evoke different learning mechanisms. Conclusions: Boundary crossers on R&D projects learn from the other professionals' tools and objectives and, in the case of transformation, integrate these in their own professional methods of working and aims. When transformation occurs school leaders and teachers develop a research attitude towards teaching and researchers incorporate contributing to educational improvement as an objective in their research. This is mainly the case in school- and researcher-directed types of cross-professional collaboration.