The most precise measurement techniques involve time, frequency, or a frequency ratio. For example, for centuries, accurate navigation has relied on precise timekeeping — a trend that continues with today’s global positioning system. After briefly reviewing the current microwave frequency standards based on the hyperfine structure of cesium, I will describe work towards atomic clocks working at optical frequencies. Among these are standards based on trapped ions or on neutral atoms trapped in an optical lattice. A frequency comb allows the comparison of different optical frequencies and the linking of optical frequencies to more-easily-counted microwave ones. Though still in the basic research stage, optical clocks have already made significant contributions to physics by setting limits for time-variation of the fundamental constants, seeing general relativistic effects at the centimeter scale, and testing local position invariance by looking for differential redshifts as the Earth moves in the Sun’s gravitational potential.