Bots are computers infected with malicious program(s) that cause them to operate against the owners’ intentions and without their knowledge. Bots communicate with and take orders from their “botmasters”. They can form distributed networks of bots, or botnets, to perform coordinated attacks. Botnets have become the platform of choice for launching attacks on the Internet, including spam, phishing, click fraud, key logging, key cracking and copyright violations, and denial of service (DoS). More ominously, botnets can be an effective malware launching platform in such a way that a new worm or virus is sent out instantaneously by numerous bots. Such lightning strike significantly shortens the response time and patch window that network administrators need to perform basic maintenance. There are many millions of bots on the Internet on any given day, organized into thousands of botnets. It is clear that botnets have become the most serious security threat on the Internet.