Intrusions are those activities that violate the security policies of a system and intrusion detection systems are used to identify those intrusions. As the networks and new technologies are growing fast new intrusions are also introduced day-by-day. Therefore supervision of communication systems get more complicated. Intrusions are detected using alerts. Correlation of alerts is by using their similar attributes, prior knowledge and corresponding vulnerabilities. In high speed networks intrusions are emerging day-by-day using different techniques. Intrusion detection methods are also available in various ways. Intrusions can be detected using graphs, using programs etc. This paper explains some of the intrusion detection methods.