With an increased understanding of how systems work, intruders have become skilled at determining weaknesses in systems and exploiting them to obtain such increased privileges that they can do anything on the system. Intruders also use patterns of intrusion that are difficult to trace and identify. They frequently use several levels of indirection before breaking into target systems and rarely indulge in sudden bursts of suspicious or anomalous activity. They also cover their tracks so that their activity on the penetrated system is not easily discovered. We must have measures in place to detect security breaches, i.e., identify intruders and intrusions. Intrusion detection systems fill this role and usually form the last line of defense in the overall protection scheme of a computer system. They are useful not only in detecting successful breaches of security, but also in monitoring attempts to breach security, which provides important information for timely countermeasures. This paper focused on how data mining is used for Intrusion detection System