A significant challenge for the design of large scale (national) eHealth systems is the fluid nature of the Patient Journey, particularly in primary healthcare modalities.In this context, a patient's journey is predisposed to organic growth rather than intelligent design. As a result, clinical tasks requiring access to high quality information may occur as a function of known healthcare patterns or as a response to a patient's state of health. Whilst Australia's national eHealth system provides the infrastructure to accommodate the self-organizing flexibility and dynamism characteristic of complex information-based ecosystems, the system neglects certain information quality attributes. Specifically, accuracy, timeliness and completeness, which may lead to improved coordination of patient care. Based on the premise that the quality of patient information plays a significant role in the performance of health professionals, consideration must be given to these information quality dimensions as an overarching goal for eHealth architectural design activities. Framed as a problem concerning the design of large scale eHealth architecture that improves the quality of health information, this perspective presents a timely research opportunity.Within a prescriptive design science research (DSR) framework, this thesis is an example of a multi-methodological approach to create and evaluate a purposeful eHealth-as-a-Service (eHaaS) design artifact, which will improve patient information quality. This was achieved by firstly, deriving abstract meta-requirements from an ethnographic examination of care pathways to establish the technical goals of the solution space. Secondly, defining the functions, organization, and structure of an eHaaS conceptual model as an example of how service-based architectures might deliver high quality information services. Finally, by establishing the validity of the conceptual model with the development of a novel evaluation strategy to explain the predicted change produced by an eHaaS design artifact.Several original contributions emerged from the research. First and foremost, the developed eHaaS conceptual model, which encapsulates a set of design principles, service-based architectural patterns and implementation strategies represents a new class of eHealth solution. One that embodies a shift away from data-centric monolithic eHealth-as-a-Service: A Service-Based Design Approach for Large Scale eHealth Architecture iii architecture to process oriented, event-driven application services. To demonstrate the benefits of this shift, an electronic patient information system (ePIMS) was developed as an original solution for orchestrating clinical information services as personalised patient workflows. Another significant contribution of this thesis is a novel evaluation strategy for validating the utility of large scale eHealth systems at the design stage. It is innovative in its use of business process and data modelling techniques (e.g. data flow diagrams, business process modelling notatio...