This paper introduces and describes an innovative modelling approach which utilises models that are synthesised through approximate calculations of user actions and extensive representation of knowledge about how to perform these actions. The Intention modelling approach is based on theories of cognitive and task modelling as well as on theories of intention, rational action and plan recognition. Intention Models (IMs) have been used in the detection of malicious attacks which usually do not consist of illegal actions, but of a set of actions individually acceptable to the system which at a higher level may form non acceptable task(s). A first effort at implementing these models for a real application was for the creation of the UII system, a research prototype for the detection of anomalous behaviour of network users obtained by reasoning about the characterisation of their intentions. It was developed as an autonomous module within SECURENET, a European funded programme that aims at...