This thesis explores the supplier perspective of supplier involvement in product development. Supplier involvement in product development is crucial for many companies to increase resource flexibility, access competence, reduce costs and time to market, among others. The existing literature focuses primarily on the customer perspective of supplier involvement, e.g., challenges and critical factors customers should consider. A few studies have focused on the supplier perspective. This research gap is addressed in this thesis by studying the conditions influencing the suppliers’ involvement in customers’ product development. This thesis explores two research questions: “What are the challenges suppliers experience when being involved in customers’ product development?”, and “What mitigating mechanisms and critical factors influence suppliers’ involvement in customers’ product development?”. The identified challenges originated [FF1] both from the customer and the supplier, indicating that both types of challenges must be focused on. The study also identified interdependencies between the challenges, implying that one challenge may lead to the occurrence of other challenges. The study suggests mitigating mechanisms to deal with the identified challenges. The identified critical factors influenced both the efficiency and effectiveness of supplier involvement in product development, e.g., lead-time, development cost, product quality, and cost. The study indicates that adopting a dual perspective and considering the challenges and critical factors for both the customer and the supplier, can support the involved supplier. The results build on findings from a case study including four SMEs.