Data mining is becoming a pervasive technology in activities as diverse as using historical data to predict the success of a marketing campaign, looking for patterns in financial transactions to discover illegal activities or analyzing genome sequences. From this perspective, it was just a matter of time for the discipline to reach the important area of computer security. This text presents a collection of research efforts on the use of data mining in computer security. Data mining has been loosely defined as the process of extracting information from large amounts of data. In the context of security, the information we are seeking is the knowledge of whether a security breach has been experienced, and if the answer is yes, who is the perpetrator. This information could be collected in the context of discovering intrusions that aim to breach the privacy of services, data in a computer system or alternatively, in the context of discovering evidence left in a computer system as part of criminal activity. The book concentrates heavily on the use of data mining in the area of intrusion detection. The reason for this is twofold. First, the volume of data dealing with both network and host activity is so large that it makes it an ideal candidate for using data mining techniques. Second, intrusion detection is an extremely critical activity. The text also addresses the application of data mining to computer forensics. This is a crucial area that seeks to address the needs of law enforcement in analyzing the digital evidence.