Spyware is a broad category of malicious software designed to intercept or take partial control of a computer's operation without the informed consent of that machine's owner or legitimate user. While the term taken literally suggests software that surreptitiously monitors the user, it has come to refer more broadly to software that subverts the computer's operation for the benefit of a third party.

Spyware differs from viruses and worms in that it does not usually self-replicate. Like many recent viruses, however, spyware is designed to exploit infected computers for commercial gain. Typical tactics furthering this goal include delivery of unsolicited pop-up advertisements; theft of personal information (including financial information such as credit card numbers); monitoring of Web-browsing activity for marketing purposes; or routing of HTTP requests to advertising sites.

As of 2005, spyware has often been described as the #1 security threat for computers running the Microsoft Windows operating systems. Some malware on the Linux and Mac OS X platforms has behavior similar to Windows spyware, but to date has not become anywhere near as widespread.