The value co-creation capability of Information Technology (IT) is a significant determinant of organisational success, but realising the value co-creation capability of IT services remains a significant challenge. The challenge stems in part from an incomplete and limited comprehension of IT service value and how it emerges within the organisation. Despite intense efforts to conceptualise the value co-creation of IT service, current approaches and frameworks only partially address the complex and dynamic nature of the IT value-creation process. This traditional and unbalanced perspective on IT service delivery mostly emphasises customer-centric practises and processes and lacks a holistic and systematic approach to understanding value co-creation that considers the multi-actor and multi-layer IT service ecosystem. With the aim of better capturing the complexities of IT service value and contributing to a more holistic, and therefore useful understanding of IT service value co-creation, the overarching research question is: "How could value co-creation be conceptualised in the IT service ecosystem?" To pursue a holistic and systematic approach to understanding IT service value co-creation, the study is framed by Service-Dominant (S-D) logic and the Service Ecosystem approach. This empirically grounded mid-range theory bridges the high-level grand theory of S-D logic to empirical findings in the IT service ecosystem. [...]