Since 1967 the primary time standard is the cesium atomic clock, based on a hyperfine transition in the microwave domain. The development of ultrastable laser sources now allows one to operate on electronic transitions in the optical domain, corresponding to a 5-order-of-magnitude increase in the clock frequency. This article reviews the spectacular accuracy and stability gains that can be obtained when working with laser cooled ions or neutral atoms. It also discusses some important applications of these optical clocks, from geodesy to tests of fundamental theories to many-body physics.

Optical atomic clocks