Organisations can use innovation to exploit new opportunities and respond to threats. Open innovation is an approach that has been shown to help organisations in managing innovation, particularly due to its emphasis on stakeholder collaboration. While the body of knowledge on the management of open innovation has developed substantially in recent years, much of this has been driven by the observation of practice in large, private sector, product-based organisations. Fewer studies have been conducted on open innovation in service-based organisations, despite the fact that economic activity in many nations is dominated by service design and delivery. In addition, much less research has investigated whether open approaches to service innovation differ between private and public sector organisations. In this dissertation, a review of the literature and an exploratory review of practice demonstrate three specific knowledge gaps. Firstly, there is a lack of a strong conceptualisation of what open innovation means for service innovation and if and how this differs from open innovation for physical products. Secondly, these reviews confirmed that the vast majority of research on innovation management focuses on private sector organisations, as is also apparent in the literature on open innovation. This is a significant point to emphasise as approaches designed for the management of innovation in the private sector cannot be assumed to be transferable to the public sector given the fundamentally different nature of these two types of organisations. Thirdly, exploratory interviews with managers in large service-focused organisations in the private and public sectors identified a strong interest in open service innovation but also revealed that they found the design of processes for developing and implementing open innovation for new services extremely challenging. To address these gaps, this study implemented a three-step methodology. First, the literature was drawn upon to develop an initial framework for conceptualising open service innovation. This framework was then used to structure the analysis of data captured from three in-depth, longitudinal qualitative case studies of the development and use of open service innovation processes in one private and two public sector organisations. Finally, the open service innovation framework was modified to reflect additional issues emerging from the case studies, which were in turn linked back to the relevant literature. This exploratory research provides insights that both build upon and, in some cases, contradict current literature on open service innovation. Specifically, the research reveals that openness in organisational culture is a particularly fundamental issue underpinning open service innovation process design. However, the results also show that co-creation does not seem to be such a dominating concept in open service innovation as previously suggested by service innovation literature, and a systematic and transparent process is a key enabler in open service innovation in the organisations analysed for this research. This study makes two main contributions. Firstly, it delivers an open service innovation framework that identifies the key elements and interdependencies for implementing open service innovation. Since the framework is an initial conceptualisation of open service innovation, it provides a platform for further research and may support practitioners in structuring the design of their open service innovation processes. Secondly, the cases reveal many commonalities between the approaches to design and use of open service innovation processes in the public and private sector organisations, but also specific features of open service innovation management in the public sector that warrant further investigation.