Byline: Marcel Bogers, Willem Horst This paper presents an inductive study that shows how collaborative prototyping across functional, hierarchical, and organizational boundaries can improve the overall prototyping process. Our combined action research and case study approach provides new insights into how collaborative prototyping can provide a platform for prototype-driven problem solving in early new product development (NPD). Our findings have important implications for how to facilitate multistakeholder collaboration in prototyping and problem solving, and more generally for how to organize collaborative and open innovation processes. Our analysis reveals two levels of prototyping. Besides the more formal managerial level, we identify the informal designer level, where the actual practice of prototyping takes place. On this level, collaborative prototyping transforms the act of prototyping from an activity belonging exclusively to the domain of design engineers to an activity integral to NPD, with participants from within the organization (different functions and managers) and from outside (consultants and users). In effect, this collapses the discrete steps in the prototyping process (at the managerial level) to an essentially continuous process of iterative problem solving (at the designer level) that is centered around the collaborative prototype, which allows participants to see their suggestions implemented and exposing them to the design constraints. The study, moreover, shows how, at various stages of the prototyping process, the actual prototype was used as a tool for communication or development, thus serving as a platform for the cross-fertilization of knowledge. In this way, collaborative prototyping leads to a better balance between functionality and usability; it translates usability problems into design changes, and it detects emerging usability problems through active engagement and experimentation. As such, the collaborative prototype acts as a boundary object to represent, understand, and transform knowledge across functional, hierarchical, and organizational boundaries. Our study also identifies some constraints in involving the appropriate stakeholders at the right time. The paper specifically elaborates on the role of users in collaborative prototyping, which is important in order to cover all phases of the problem-solving cycle but triggers an interesting challenge due to the 'reverse empathy' that a user may develop for the design constraints-parallel to the designer empathy for the user context. Finally, our study shows that despite the continuous nature of the (designer) practice of prototyping, there are certain windows of opportunities (at the managerial level) during which the collaborative prototyping approach actually leads to changes in the product design. Biographical information: Dr. Marcel Bogers is an associate professor of innovation and entrepreneurship at the University of Southern Denmark. He received his Ph.D. in management of technology from the Ecole Polytechnique Federale de Lausanne (Swiss Federal Institute of Technology). He also studied at the Eindhoven University of Technology, University of California at Berkeley, and Chalmers University of Technology. His main interests center around the design, organization, and management of technology and innovation. More specifically, he has studied areas such as business models, open innovation, users as innovators, collaborative prototyping, entrepreneurship, improvisation, and learning-by-doing. Dr. Willem Horst is a user experience software producer at LEGO and is also a freelance interaction design consultant. He received his Ph.D. in design and innovation from the University of Southern Denmark, and has a background in industrial design (BSc) and IT product design (MSc). His research focuses on the potential of prototypes as platforms for participation across different stakeholders in new product development. He has been involved in new product development projects at Philips, LEGO, and Danfoss to develop a collaborative approach to prototyping interactive products. Article Note: The authors would like to thank the people at Danfoss Heating Solutions who were involved in the project described in this paper for their support during the project and their cooperation afterwards. Special thanks go to Anders Ostergaard Clausen for his support and engagement throughout the process. The authors are also grateful for the editor's guidance and the constructive reviewer comments, which helped improve this paper.