Today''s age of universal electronic connectivity is also the age of viruses, worms, bots, trojan horses, trapdoors, rabbits, root kits and hackers. It is the age of eavesdropping and electronic fraud. Exponential growth in computer crimes is plaguing the digital information systems all around our globe. This highlights the importance of Information Security in the current era, due to an outsized variety of cyber threats. Past several year''s research has introduced many dimensions to detect malicious attacks in order to secure system resources and data. One of these dimensions is "Intrusion Detection System" (IDS). This book studies the beginning of detection, analysis and reporting of malicious activity, where it is today and where it appears to be heading in the future. It also elucidates the intrusive techniques used by intruders, their activities and also the vulnerabilities in computing systems that enable them. It explains the chronological summary of the "Intrusion Detection field" with an in-depth vision of the involved technologies "Taxonomy of IDSs". It exposes the IDSs detection methods, audit sources, usage frequencies and their behavior on intrusion detection.