In this chapter, we will examine the technical issues surrounding rogue code, now referred to generally as malware (malicious software). The major types of malware we will discuss are viruses, Trojan horses, worms, root kits, spyware, and botnets. We will describe these types of rogue code, as well as logic bombs, and techniques for examination of systems after an intruder’s attack. We also will discuss detection and prevention methods on various types of mission-critical computers. Since the original publication of this book, the topic of malware has become the leading security topic of discussion. Thus, given that investigators will deal with rogue code extensively, we offer this chapter as one of the more important ones in the book.