This paper aims to analyse the practice of knowledge integration (KI) in Collaborative New Product Development (Co-NPD) projects involving multiple partners in an aerospace network. On the basis of a pragmatic approach, a multiple-case study is developed focusing on two Italian aerospace companies involved in a research project aimed to define methodologies and technologies for knowledge collaboration in NPD. Data are gathered with a triangulation technique merging official documents and open-ended and closed-ended interviews administered in the companies’ sites and involving managers, engineers and employees. The paper proposes a systematic treatment of the KI practice describing the types of knowledge, strategies, barriers and intellectual property policies for KI at inter-organizational level. The findings provide insights to address NPD models for aerospace networks where the complexity and value of codified knowledge coming from external sources requires adequate treatments and security standards.