  The number of known millisecond pulsars has dramatically increased in the
last few years. Regular observations of these pulsars may allow gravitational
waves with frequencies ~10^-9 Hz to be detected. A ``pulsar timing array'' is
therefore complimentary to other searches for gravitational waves using
ground-based or space-based interferometers that are sensitive to much higher
frequencies. In this review we describe 1) the basic methods for using an array
of pulsars as a gravitational wave detector, 2) the sources of the potentially
detectable waves, 3) current limits on individual sources and a stochastic
background and 4) the new project recently started using the Parkes radio
telescope.
