  Pulsar timing is a technique that uses the highly stable spin periods of
neutron stars to investigate a wide range of topics in physics and
astrophysics. Pulsar timing arrays (PTAs) use sets of extremely well-timed
pulsars as a Galaxy-scale detector with arms extending between Earth and each
pulsar in the array. These challenging experiments look for correlated
deviations in the pulsars' timing that are caused by low-frequency
gravitational waves (GWs) traversing our Galaxy. PTAs are particularly
sensitive to GWs at nanohertz frequencies, which makes them complementary to
other space- and ground-based detectors. In this chapter, we will describe the
methodology behind pulsar timing; provide an overview of the potential uses of
PTAs; and summarise where current PTA-based detection efforts stand. Most
predictions expect PTAs to successfully detect a cosmological background of GWs
emitted by supermassive black-hole binaries and also potentially detect
continuous-wave emission from binary supermassive black holes, within the next
several years.
