As network-based computer systems play increasingly vital roles in modern society, they have become the targets of criminals. Network security has never been more important a subject than in today's extensively interconnected computer world. Intrusion Detection Systems (IDS) have been used along with the data mining techniques to detect intrusions. In this thesis, we present a comparative study of intrusion detection using a decision-tree learner (C4.5), two rule-based learners (ripper and ridor), a learner to combine decision trees and rules (PART), and two instance-based learners (IBK and Nnge). We investigate and compare the performance of IDSs based on the six techniques, with respect to a case study of the DAPAR KDD-1999 network intrusion detection project. Investigation results demonstrated that data mining techniques are very useful in the area of intrusion detection.