With our society's growing dependency on information technology systems (IT), the issue of IT Security becomes increasingly important. IT security cannot be achieved by means of preventive safeguards alone. To properly respond to misuse or abusive activity in IT systems, one needs to establish the capability to detect and understand improper activity.  Intrusion Detection Systems (IDSs) observe activity occurring in the IT system, record these observations in audit data, and analyze the collected audit data in order to detect misuse.