  Pulsar timing array projects are carrying out high precision observations of
millisecond pulsars with the aim of detecting ultra-low frequency (~ 10^{-9} to
10^{-8} Hz) gravitational waves. We show how unambiguous detections of such
waves can be obtained by identifying a signal that is correlated between the
timing of different pulsars. Here we describe the ongoing observing projects,
the expected sources of gravitational waves, the processing of the data and the
implications of current results.
