Purpose - The purpose of this paper is to explore and characterize knowledge integration approaches for integrating external knowledge of suppliers into new product development projects. Design/methodology/approach - This paper is based on a multiple, in-depth case study of six product development projects at three knowledge-intensive manufacturing firms. Findings - Firms make purposeful choices to devise knowledge integration approaches when working in collaborative buyer - supplier projects. The knowledge characteristics of the supplier input guide the choice of either coupling knowledge sharing and combining across firms or decoupling knowledge sharing (across firms) and knowledge combining (within firms). Research limitations/implications - This study relies on a limited number of case studies and considers only one supplier relationship in each project. Further studies could examine the challenge of knowledge integration in buyer - supplier relationships in different contexts, i.e. in relation to innovation complexity and uncertainty. Practical implications - Managers need to make choices when designing knowledge integration approaches in collaborative product development projects. The use of coupled and decoupled approaches can help balance requirements in terms of joint problem-solving across firms, the efficiency of knowledge integration and the risks of knowledge leakage. Originality/value - The conceptualization of knowledge integration as knowledge sharing and knowledge combining extends existing perspectives on knowledge integration as either a transfer of knowledge or as revealing the presence of pertinent knowledge without entirely transmitting it. The findings point to the complexity of knowledge integration as a process influenced by knowledge characteristics, perspectives on knowledge, openness of firm boundaries and elements of knowledge sharing and combining.