The purpose of this chapter is to provide a wide spectrum of end users with a complete reference on malicious code or malware. End users include researchers, students, as well as information technology and security professionals in their daily activities. A particular effort aims at educating users about malware, enhancing organization capabilities for preventing as well as handling malicious code incidents when they occur, and preparing them for tomorrow’s new types of malware, as well as the new types of safeguards they should consider. First, the author provides an overview of malicious code, its past, present, and future. Second , he presents methodologies , guidelines and recommendation on how an organization can enhance its prevention of malicious code, how it should respond to the occurrence of a malware incident, and how it should learn from such an incident to be better prepared in the future. Finally, the author addresses the issue of the current research as well as future trends of malicious code and the new and future means of malware prevention.