  In the last ten years extraordinary results in time and frequency metrology
have been demonstrated. Frequency-stabilization techniques for continuous-wave
lasers and femto-second optical frequency combs have enabled a rapid
development of frequency standards based on optical transitions in ultra-cold
neutral atoms and trapped ions. As a result, today's best performing atomic
clocks tick at an optical rate and allow scientists to perform high-resolution
measurements with a precision approaching a few parts in $10^{18}$. This paper
reviews the history and the state of the art in optical-clock research and
addresses the implementation of optical clocks in a possible future
redefinition of the SI second as well as in tests of fundamental physics.
