While the importance of internal integration for effective inter-firm collaboration with suppliers is widely acknowledged, it is presently unclear how it is achieved in complex collaborative product development projects. This paper aims to address this gap in extant knowledge by investigating the internal integration approaches and exploring related project management challenges. Specifically, three internal integration approaches are found, namely integration based on multidirectional, frequent interaction; integration based on delimited, problem-solving; and based on unidirectional, information-oriented interaction. The study findings suggest that internal integration approaches are related to the degree of uncertainty in the subsystems of the suppliers, rather than the overall product system. Consequently, in complex product development projects involving many internal functions and several different suppliers, the specific supplier tasks, rather than the overall project structure and aims, determine the mode of internal integration required. This complexity creates important challenges for organisation, and requires flexibility in internal integration approaches.