This paper analyses the knowledge sourcing, transformation, and exploitation stages of the innovation value chain for a sample of Irish small to medium sized enterprises (SMEs) using Community Innovation Survey data. It explores the role of internal research and development (R&D) and external knowledge on SMEs’ innovation and performance. The open innovation paradigm, which stresses the importance of external linkages, is used to examine the impact of different external knowledge sources on SMEs’ innovation. The consideration of external linkages in the innovation performance of SMEs is crucial as these firms may be constrained in their ability to perform R&D due to their size. The analysis expands the traditional CDM methodology beyond the consideration of research and development as the sole source of knowledge for innovation by also considering a range of potential external knowledge sources. The findings indicate that SMEs generate knowledge internally through the performance of R&D, while also exploiting linkages to external agents. However, the impact of external sources of knowledge is not uniform. The results suggest that backward linkages have a positive impact on SME product innovation, but negatively affect SME process innovation, while public knowledge sources are positively related to the probability of product innovation occurring. This may have important policy implications. Finally, process innovation is also found to be a key determinant of SME productivity, while product innovation has no impact on SME performance.