  The phenomenal rotational stability of millisecond pulsars allows them to be
used as precise celestial clocks. An array of these pulsars can be exploited to
search for correlated perturbations in their pulse times of arrival due to
gravitational waves. Here, I describe the observations and analysis necessary
to accomplish this goal and present an overview of the efforts of the worldwide
pulsar timing community. Due to a growing number of millisecond pulsar
discoveries, improved instrumentation, and growing timespans of observation,
the sensitivity of our pulsar timing array experiments is expected to
dramatically increase over the next several years, leading to either a
gravitational wave detection or very stringent constraints on low-frequency
gravitational wave source populations before the end of the decade.
