Intruction detection system In the last three years, the networking revolution has finally come of age. More than ever before, we see that the Internet is changing computing as we know it. The possibilities and opportunities are limitless; unfortunately, so too are the risks and chances of malicious intrusions. It is very important that the security mechanisms of a system are designed so as to prevent unauthorized access to system resources and data. However, completely preventing breaches of security appear, at present, unrealistic. We can, however, try to detect these intrusion attempts so that action may be taken to repair the damage later. This field of research is called Intrusion Detection. Anderson, while introducing the concept of intrusion detection in 1980, defined an intrusion attempt or a threat to be the potential possibility of a deliberate unauthorized attempt to" access information, " manipulate information, or " render a system unreliable or unusable. Since then, several techniques for detecting intrusions have been studied. This paper discusses why intrusion detection systems are needed, the main techniques, present research in the field, and possible future directions of research