Keystroke logging, often referred to as keylogging or keyboard capturing, is the action of recording (logging) the keys pressed on a keyboard, typically covertly, so that a person using the keyboard is unaware that their actions are being monitored. Data can then be retrieved by the person operating the logging program. A keystroke recorder or keylogger can be either software or hardware.

While the programs themselves are legal, with many designed to allow employers to oversee the use of their computers, keyloggers are most often used for stealing passwords and other confidential information. Keystroke logging can also be utilized to monitor activities of children in schools or at home and by law enforcement officials to investigate malicious usage.

Keylogging can also be used to study keystroke dynamics or human-computer interaction. Numerous keylogging methods exist, ranging from hardware and software-based approaches to acoustic cryptanalysis.

In the mid-1970s, the Soviet Union developed and deployed a hardware keylogger targeting US Embassy typewriters. Termed the "selectric bug", it transmitted the typed characters on IBM Selectric typewriters via magnetic detection of the mechanisms causing rotation of the print head. An early keylogger was written by Perry Kivolowitz and posted to the Usenet newsgroup net.unix-wizards, net.sources on November 17, 1983. The posting seems to be a motivating factor in restricting access to /dev/kmem on Unix systems. The user-mode program operated by locating and dumping character lists (clients) as they were assembled in the Unix kernel.

In the 1970s, spies installed keystroke loggers in the US Embassy and Consulate buildings in Moscow. They installed the bugs in Selectric II and Selectric III electric typewriters.

Soviet embassies used manual typewriters, rather than electric typewriters, for classified information—apparently because they are immune to such bugs. As of 2013, Russian special services still use typewriters.

A software-based keylogger is a computer program designed to record any input from the keyboard. Keyloggers are used in IT organizations to troubleshoot technical problems with computers and business networks. Families and businesspeople use keyloggers legally to monitor network usage without their users' direct knowledge. Microsoft publicly stated that Windows 10 has a built-in keylogger in its final version "to improve typing and writing services". However, malicious individuals can use keyloggers on public computers to steal passwords or credit card information. Most keyloggers are not stopped by HTTPS encryption because that only protects data in transit between computers; software-based keyloggers run on the affected user's computer, reading keyboard inputs directly as the user types.

From a technical perspective, there are several categories: