Collaboration between practitioners and researchers can increasingly be observed in research and development (R&D) projects in secondary schools. This article presents an analysis of cross-professional collaboration between teachers, school leaders and educational researchers and/or advisers as part of R&D projects in terms of three dimensions: reasons for collaboration, the division of roles and tasks, and the communication structure. Data were collected in interviews with participants from 12 Dutch projects, from documents and from meetings with participants. The results show that the parties involved can have the same reasons for cross-professional collaboration but that the external educational experts may also have additional objectives. Roles and tasks were divided in different ways, depending on who directs and guides the research and development, and whether teachers were involved as teacher researchers in the project. The communication structure appeared to be linked to the division of roles and tasks, but also to the number of participants in the project. The three dimensions proved to be valuable viewpoints to study cross-professional collaboration in R&D projects in Dutch secondary education. Furthermore, we distinguished four types of cross-professional collaboration in R&D projects. These types differ in the extent to which the school or the external party direct and guide the project: (a) School-directed collaboration; (b) School- and researcher-directed collaboration; (c) School- and adviser-directed collaboration; (d) Researcher-directed collaboration. Our findings can support participants to consciously choose a particular type of cross-professional collaboration that fits their purposes and specific situation.