An information system was previously defined as the socio-technical subsystem of an institution, which comprises all information processing as well as the associated human or technical actors in their respective information processing roles. Health information systems (HIS) are dealing with processing data, information, and knowledge in health care environments. Especially with regard to chronic diseases, it becomes more and more important to organize health care in a patient-centric way, such that all participating in- or outpatient care institutions cooperate very closely. This is also denoted as integrated care. In integrated care it is necessary to provide relevant information not only within a single institution, but wherever and whenever it is needed. This includes medical practices, rehabilitation centers, nursing centers, and even the home of the patient. We therefore differentiate institutional and transinstitutional health information systems. In the following, we will introduce hospital information systems, which are the most complex instances of institutional information systems, and transinstitutional information systems. Throughout the book we will use the term health information system and the abbreviation HIS if we discuss aspects concerning both hospital information systems and transinstitutional information systems. If we deal with properties being unique for one of these, we will use the terms hospital information system and transinstitutional information system, respectively.