Standards of Time and Frequency at the Outset of the 21st Century

After 50 years of development, microwave atomic clocks based on cesium have achieved fractional uncertainties below 1 part in 1015, a level unequaled in all of metrology. The past 5 years have seen the accelerated development of optical atomic clocks, which may enable even greater improvements in timekeeping. Time and frequency standards with various levels of performance are ubiquitous in our society, with applications in many technological fields as well as in the continued exploration of the frontiers of basic science. We review state-of-the-art atomic time and frequency standards and discuss some of their uses in science and technology.