Defamation is an area of tort law that makes it illegal for an individual to make a false statement that tends to harm another individual’s reputation (Schimmel, Stellman & Fischer, 2010). A statement can be defamatory only if it is communicated to a third person (Schimmel, Stellman, & Fischer, 2010). There are two types of defamation. Slander applies to oral statements, while libel applies to written statements. Because emails, tweets and instant messaging are written statements, they fall under the category of libel (Burke, 2011). This chapter provides an overview of the law of defamation and its application to off-campus utterances by students. The first part discusses United States Supreme Court cases that define the modern parameters of defamation law. The second part discusses the Communication Decency Act (CDA) and the definitive case on CDA, Zeran v. American Online Inc. (1997). The final part discusses cases that have addressed defamation with respect to student online communications.