  Pulsars are wonderful gravitational probes. Their tiny size and stellar mass
give their rotation periods a stablility comparable to that of atomic frequency
standards. This is especially true of the rapidly rotating "millisecond
pulsars" (MSPs). Many of these rapidly rotating pulsars are in orbit with
another star, allowing pulsar timing to probe relativistic perturbations to the
orbital motion. Pulsars have provided the most stringent tests of theories of
relativistic gravitation, especially in the strong-field regime, and have shown
that Einstein's general theory of relativity is an accurate description of the
observed motions. Many other gravitational theories are effectively ruled out
or at least severely constrained by these results. MSPs can also be used to
form a "Pulsar Timing Array" (PTA). PTAs are Galactic-scale interferometers
that have the potential to directly detect nanohertz gravitational waves from
astrophysical sources. Orbiting super-massive black holes in the cores of
distant galaxies are the sources most likely to be detectable. Although no
evidence for gravitational waves has yet been found in PTA data sets, the
latest limits are seriously constraining current ideas on galaxy and black-hole
evolution in the early Universe.
