  Millisecond pulsars are intrinsically very stable clocks and precise
measurement of their observed pulse periods can be used to study a wide variety
of astrophysical phenomena. In particular, observations of a large sample of
millisecond pulsars at regular intervals, constituting a Pulsar Timing Array
(PTA), can be used as a detector of low-frequency gravitational waves and to
establish a standard of time independent of terrestrial atomic timescales.
Three major timing array projects have been established: The European Pulsar
Timing Array (EPTA), the North American pulsar timing array (NANOGrav) and the
Parkes Pulsar Timing Array (PPTA). Results from the PPTA project are described
in some detail and future prospects for PTA projects are discussed.
