© 2017 The Authors. The term Integrated Product Development (IPD) has been introduced as a focus for cross-disciplinary research and can have several forms, or manifestations, with regard to the existing disciplines such as concurrent engineering and design for manufacturing. Of central importance to IPD is the interpretation of the term "integration", particularly with regard to internal and external elements. However, there is not yet an explicit understanding of an appropriate degree of integration, or involvement, with respect to its different forms, that can assure successful implementation of IPD frameworks in practice. Through a review and clustering of the literature, this paper aims to address this challenge.