  Pulsar Timing Array (PTA) experiments exploit the clock-like behaviour of an
array of millisecond pulsars, with the goal of detecting low-frequency
gravitational waves. PTA experiments have been in operation over the last
decade, led by groups in Europe, Australia, and North America. These
experiments use the most sensitive radio telescopes in the world, extremely
precise pulsar timing models and sophisticated detection algorithms to increase
the sensitivity of PTAs. No detection of gravitational waves has been made to
date with this technique, but PTA upper limits already contributed to rule out
some models of galaxy formation. Moreover, a new generation of radio
telescopes, such as the Five hundred metre Aperture Spherical Telescope and, in
particular, the Square Kilometre Array, will offer a significant improvement to
the PTA sensitivity. In this article, we review the basic concepts of PTA
experiments, and discuss the latest results from the established PTA
collaborations.
