Driven by rapid technological developments, greater customer expectations, and increased product complexity, product development processes increasingly rely on the integration of dispersed specialist knowledge. Consequently, many industrial firms are changing the way they approach product development. Product development increasingly relies on knowledge integration across firm boundaries. Customers are one of the most indispensable sources of knowledge, and their knowledge needs to be integrated during product development. However, little is known about the processes and mechanisms used to integrate customer knowledge in product development. Therefore, firms must devise processes and mechanisms that support knowledge integration with customers. This thesis aims to investigate knowledge integration with customers in collaborative product development.The work described in this thesis combines a case study with a survey approach. More specifically, five case studies were conducted at three industrial manufacturing firms. Data were collected in 46 interviews, along with informal discussions, workshops, and secondary material. The case studies revealed the role of knowledge integration, knowledge integration mechanisms, and the importance of contextual factors related to the customer's technical capability, and the locus of initiative for the product development project. A cross-sectional survey was performed in order to ascertain the role of knowledge integration and its effect on innovation performance and the effect of firms and customers' technical capability and locus of initiative on knowledge integration with customers. The survey study generated 216 responses from firms operating in the Swedish manufacturing industry.This thesis consists of a compiled summary and five appended papers. The results in these provide the basis for modelling knowledge integration with customers. The findings confirm that integrating knowledge with customers in product development is associated with better innovation performance. Further, the results point to the dynamic characteristics of knowledge integration in product development processes. More specifically, firms tend to apply different combinations of mechanisms in different phases, depending on the intended content of the knowledge contribution of the customer and the requirements of each phase in the development process. In addition, the degree of technical capability, internal integration capability, and locus of initiative are important factors influencing knowledge integration processes. This thesis sheds new light on collaborative product development with customers by demonstrating that knowledge, and particularly the integration of knowledge across firm boundaries, is a central property of product development. The study focuses on industrial and manufacturing firms, complementing current perspectives on the role of customers in other sectors. In this particular sector, complexity and system-wide implications need to be resolved together with customers through ...