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.