  Optical frequency combs were developed nearly two decades ago to support the
world's most precise atomic clocks. Acting as precision optical synthesizers,
frequency combs enable the precise transfer of phase and frequency information
from a high-stability reference to hundreds of thousands of tones in the
optical domain. This versatility, coupled with near-continuous spectroscopic
coverage from the terahertz to the extreme ultra-violet, has enabled precision
measurement capabilities in both fundamental and applied contexts. This review
takes a tutorial approach to illustrate how 20 years of source development and
technology has facilitated the journey of optical frequency combs from the lab
into the field.
