This chapter covers what antivirus (AV) software is and how it works. It offers a brief history of AV software and a short analysis of how it evolved over time. The chapter talks about the various components of an AV, namely, the kernel, command‐line scanner, graphical user interface (GUI) scanner, daemons or system services, file system filter drivers, network filter drivers, and any other support utility that ships with it. An AV product may not always offer third‐party developers direct access to its core; instead, it may offer access to command‐line scanners. Most AV users believe that security products are bulletproof and that just installing AV software keeps their computers safe. The chapter enumerates and briefly discusses some common features shared by most AV products, as well as more advanced features that are available only in some products.