  Pulsars are very stable clocks in space which have many applications to
problems in physics and astrophysics. Observations of double-neutron-star
binary systems have given the first observational evidence for the existence of
gravitational waves (GWs) and shown that Einstein's general theory of
relativity is an accurate description of gravitational interactions in the
regime of strong gravity. Observations of a large sample of pulsars spread
across the celestial sphere forming a "Pulsar Timing Array" (PTA), can in
principle enable a positive detection of the GW background in the Galaxy. The
Parkes Pulsar Timing Array (PPTA) is making precise timing measurements of 20
millisecond pulsars at three radio frequencies and is approaching the level of
timing precision and data spans which are needed for GW detection. These
observations will also allow us to establish a "Pulsar Timescale" and to detect
or limit errors in the Solar System ephemerides used in pulsar timing analyses.
Combination of PPTA data with that of other groups to form an International
Pulsar Timing Array (IPTA) will enhance the sensitivity to GWs and facilitate
reaching other PTA goals. The principal source of GWs at the nanoHertz
frequencies to which PTAs are sensitive is believed to be super-massive binary
black holes in the cores of distant galaxies. Current results do not
signficantly limit models for formation of such black-hole binary systems, but
in a few years we expect that PTAs will either detect GWs or seriously
constrain current ideas about black-hole formation and galaxy mergers. Future
instruments such as the Square Kilometre Array (SKA) should not only detect GWs
from astrophysical sources but also enable detailed studies of the sources and
the gravitational theories used to account for the GW emission.
