  Pulsars are amongst the most stable rotators known in the Universe. Over many
years some millisecond pulsars rival the stability of atomic clocks. Comparing
observations of many such stable pulsars may allow the first direct detection
of gravitational waves, improve the Solar System planetary ephemeris and
provide a means to study irregularities in terrestrial time scales. Here we
review the goals and status of current and future pulsar timing array projects.
