A considerable number of formal information security models have been developed during the last two decades. We present and discuss some of the most widespread ones that have been successfully applied to the traditiona~ centralised Information Systems of the past. We show the special security needs of modern information systems that are based on the concepts of Open Distributed Processing, the Object-oriented paradigm and multimedia technology. We argue that these Information Systems need new

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... enhanced information security models in order to address the information security issue effectively and present some efforts towards this goal Keywords Information Security, Information Systems Security, Formal Information Security Models. P. Horster (ed.), Communications and Multimedia Security II © IFIP International Federation for Information Processing 1996 Need for information security models 257 to a particular set of purposes", where IT Assembly is "a collection of computer hardware, software, (and sometimes communication equipment or other IT components) capable of being used to handle information". The importance of communication equipment has lead to the use of the more complete term Information Technology and Communications Security. Information Systems (IS) Security, on the other hand, has a broader meaning. Schoderbek (Schoderbek, 1990 ) defines the term system as follows: "a system is a set of objects together with relationships between the objects and between their attributes related to each other and to their environment so as to form a whole." The term Information System can be defined as a system that handles information and consists of 5 interrelated elements, namely hardware, software, data/information, people and procedures (Kiountouzis et al., 1996) . So, we may say that IS Security is the scientific discipline that deals with the problem of protecting the five elements that consist an IS and the IS as a whole.