HISA specifies fundamental requirements for 'information infrastructure' and healthcare specific middleware services. This part of the standard specifies the fundamental characteristics of the computational model to be implemented by a specific architectural layer of the information system (i.e. the middleware) to provide a comprehensive and integrated interface to the common enterprise information and to support the fundamental business processes of the healthcare organisation, as defined in the document “Health Informatics – Service Architecture - Part 1: Enterprise Viewpoint”. The computational model is specified without any –explicit or implicit- assumption about the physical technologies, tools or solutions to be adopted for its physical implementation in the various target scenarios. The specification is nevertheless formal, complete and non-ambiguous enough to allow implementers to derive an efficient design of the system in the specific technological environment that will be selected for the physical implementation. The computational model provides the basis for ensuring consistency between different engineering and technology specifications (including programming languages and communication mechanisms) since they must be consistent with the same computational object model. This consistency allows open inter-working and portability of components in the resulting implementation. This specification does not aim at representing a fixed, complete, specification of all possible interfaces that may be necessary for any requirement of any healthcare enterprise. It specifies only a set of characteristics –in terms of overall organisation and individual computational objects, identified as fundamental and common to all healthcare organisations, and that are satisfied by the computational model implemented by the middleware.