This chapter discusses viruses, worms, and Trojan Horses; and further explores their differences and similarities. Computer viruses are just one example of what is commonly referred to as malicious code, or malicious programs. Malicious programs are created to perform a series of harmful actions on a computer system. For a computer system to become infected with a virus, it must come into contact with a carrier of the virus. For this discussion two terms are defined: virus activation and virus execution. Once an infected program is executed or an infected operating system performs a task, the virus is activated. As infection can come from many places, a comprehensive set of protective policies should be established and adhered to. As with virus prevention, Trojan Horse prevention can be largely accomplished by enforcing a strict software installation policy and educating users. Trojan Horse removal is easier as viruses propagate, thus necessitating extensive system examination and eradication.