In an increasing number of security threats, it is a severe challenge to protect target system, developing flexible and adaptive security oriented approaches against malicious activities in network. Intrusion detection is the act of detecting, monitoring unwanted activity and traffic on a network or a device, which violates security policy.This paper begins with a review of the most well-known anomaly based intrusion detection techniques. AIDS is a system for detecting computer intrusions, type of misuse that falls out of normal operation by monitoring system activity and classifying it as either normal or anomalous. It is based on heuristics or rules of artificial intelligence and neural networks techniques, rather than the pattern or signature. This is opposed to signature based system. Finally, dealt with wide anomaly based intrusion detection issue on assessment.