Horology (from Greek: ὥρα, "hour, time" and Greek: λόγος, logos, "study, speech"; lit. the study of time) is the art or science of measuring time. Clocks, watches, clockwork, sundials, clepsydras, timers, time recorders and marine chronometers are all examples of instruments used to measure time.

People interested in horology are called horologists. That term is used both by people who deal professionally with timekeeping apparatus (watchmakers, clockmakers), as well as aficionados and scholars of horology. Horology and horologists have numerous organizations, both professional associations and more scholarly societies.

Horology has a long history^{[chronology citation needed]} and there are many museums and several specialized libraries devoted to the subject. An example is the Royal Greenwich Observatory, which is also the source of the Prime Meridian (longitude 0° 0' 0"), and the home of the first marine timekeepers accurate enough to determine longitude (made by John Harrison). Other horological museums in the London area include the Clockmakers' Museum, and the horological collections at the British Museum, the Science Museum (London) and at the Wallace Collection.