This study addresses the phenomenon of orchestration, defined as the long-term mobilization, convergence, and arrangement of actors in the development of an inter-organizational information system for integrating and sharing data across a regional care ecosystem in England. Using multi-sited longitudinal qualitative research, I analyze the first five years (2014-2019) of a novel, regional interoperability platform in England's National Health Service (NHS), intended to interconnect primary, secondary, and social care information systems for the integration and exchange of clinical data. This region was an early mover in the implementation of new policies to promote integrated care and information systems interoperability at regional level. Experiences like the National Programme for Information Technology (NPfIT) (2002-2011) in NHS England have demonstrated that the scale and complexity of the NHS make the implementation of one-size-fits-all, nation-wide, centralized digital solutions difficult. After NPfIT was abandoned in 2011, hospitals were free to choose and implement the Electronic Medical Records (EMR) system they believed best matched their requirements. Diverse stand-alone computer systems were thus implemented under this laissez-faire policy. This has contributed to the separate digitalization of primary, secondary, and social care services and reinforced the fragmentation and dispersion of data. Since 2014, increasing priority has been given to integrating care, with health policies emphasizing the efficient processing of patients and patient data, and integrating information to support entire care pathways. This has created a new impetus to bring previously separated systems together through shared information infrastructures. Avoiding nationwide projects, such efforts have been promoted through region-wide approaches initiated and governed by participant organizations. Heterogenous groups of autonomous organizations must mobilize over the long-term to develop - conceptualize, design, implement, adopt, and post-implement - an inter-organizational digital infrastructure. These efforts towards information systems interoperability at regional level, present a challenging sociotechnical dynamic. In these long-term constructions of digital information infrastructures, governance is neither top-down nor bottom-up, participation is not mandated, and the roles of the organizations involved are unclarified. Development is shaped by in-the-making design and use, the multiplicity of agendas, contexts, and user bases, the installed base, and existing systems and practices that restrict design scope. At the inter-organizational level, the process also involves links between subsystems, a diversity of actors, a dispersion of knowledge, and diverse and asymmetrical installed bases. In situations such as these, involving autonomous and interrelated organizations and where knowledge and authority do not rest on a single actor, long-term mobilization and participation become critical, and traditional command and control dynamics are ineffective. Therefore, stakeholders are required to find ways to coordinate in a non-hierarchical manner. My research contributes to an understanding of orchestration in the development of a novel, inter-organizational information infrastructure, within a context where authority, knowledge, and expertise do not lie in a single entity or person. This empirical research focuses on an early mover in materializing the mandate of NHS England for regional inter-organizational information systems interoperability. Close to Greater London, this English region covers an area of approximately 500 square miles and a population of 1 000 000 inhabitants. The region includes two Hospital Trusts, one large Mental Health Hospital Trust, over 100 GP surgeries and six Local Authority providers of social care services. Through longitudinal, multisite qualitative research, including interviews with implementors, non-participant observation, and document analysis, my research provides a detailed analysis of the early development of this inter-organizational system, from the initial mobilization of actors to early use; an in-depth understanding of how the long-term interplay between the convergence of actors, recurrent interpretations of the vision of integration, the configuration of governance, and experimentation as a way of learning have shaped the development of this novel information infrastructure; a broad understanding of the implications of multiple installed bases of organizations with different digitalization trajectories and systems in this development; and, early insights into use of the inter-organizational system in various contexts. This study illustrates that the orchestration of autonomous actors requires a vision of integration that is often ambiguous, enabling participants to constantly reinterpret the vision as the development of the new system progresses and new possibilities arise. I highlight the interplay of attributes that can support the initial mobilization of actors prior to implementation. Initial "calls" for the development of an inter-organizational information infrastructure may require a combination of elements, including economic resources, legitimate interest, authority, IT expertise at scale, and local knowledge of the ecosystem's dynamics. As these attributes are unlikely to be combined within a single actor, established alliances are necessary to promote comprehensive mobilization. This study reveals that the dynamics of collective learning, the legitimation of decisions and actions through consultation and engagement, the conciliation of tensions, the reconfiguration and evolution of governance, and experimentation in the making all had an impact on this regional project, shaping the long-term development of this novel, inter-organizational information infrastructure. One way this research contributes to the literature on information infrastructures and innovation ecosystems is by introducing the concept of Orchestration Space. I define this as the cognitive learning space in which the process of developing an inter-organizational information infrastructure is shaped. Two reciprocal forces converge in an Orchestration Space: the constant, long-term mobilization and convergence of actors towards a vision-in-progress that helps drive infrastructure development; and the momentum, traction, collective knowledge, and experience that contributes to mobilizing and maintaining the investment of actors as development progresses. Understanding the nature, emergence, and evolution of an Orchestration Space and its dynamics is crucial for the development and governance of inter-organizational information infrastructures.