The basic experiment exploits the predictability of the times of arrival of pulses from millisecond pulsars and uses them as a system of Galactic clocks. Disturbances in the clocks will be measurable at Earth. A disturbance from a passing gravitational wave will have a particular signature across the ensemble of pulsars, and will be thus detected. The experiment is analogous to ground-based interferometric detectors such as LIGO and VIRGO, where the time-of-flight of a laser beam is measured along a particular path and compared to the time-of-flight along an orthogonally oriented path. Instead of the time-of-flight of a laser beam the IPTA is measuring the time-of-flight of an electromagnetic pulse from the pulsar. Instead of 4 km arms the 'arms' of the IPTA are thousands of light-years Each of the PTAs times approximately 20 MSPs each month. With significant overlap between the collaborations the total number of MSPs timed by the IPTA is approximately 30. These differences between the IPTA and the ground-based interferometers allow them to probe a completely different range in gravitational-wave frequency and thus a different category of sources. Whereas ground-based detectors are sensitive to between tens and thousands of Hz, the IPTA is sensitive to between tens and hundreds of microHertz. Their primary source of gravitational waves is supermassive black-hole binaries, presumed to exist in plenty in the universe at the centers of galaxies, resulting from previous mergers of those galaxies. The resources of the IPTA are substantial. The EPTA uses large quantities of time on Europe's five 100-meter class telescopes: the Lovell Telescope at Jodrell Bank, England, the Effelsberg 100-m Radio Telescope in Germany, the Sardinia Radio Telescope in Italy, the Westerbork Synthesis Radio Telescope in the Netherlands, and the Nancay Radio Telescope in France. Together these 5 telescopes make up the Large European Array for Pulsars in which they operate together as a single 300-meter class telescope. NANOGrav uses about 1-day per month of time at the Green Bank 100-m telescope, and 0.5 days per month at the 300-m Arecibo Observatory in Puerto Rico. The PPTA uses several days per month at the 64-meter Parkes Radio Telescope. Pulsar timing was tied for top ranking in the "medium size" category for priorities from the Particle Astrophysics and Gravitational Panel of the Astro2010 Decadal Review sponsored by the National Academy. The IPTA is coordinated and advised by the IPTA Steering Committee, a seven-member committee with two representatives from each of the three PTA consortia plus the immediate past Chair. Currently on the committee are Dick Manchester, Willem van Straten, Scott Ransom, Ingrid Stairs, Ben Stappers, Gilles Theureau, and Andrea Lommen. Each of the three consortia are also members of the Gravitational Wave International Committee, an advisory councilconsisting of the leaders of gravitational wave experiments worldwide. The first IPTA data release was on the 12 February 2016, which provided a 2 sigma limit on the amplitude of the Gravitational Wave Background.
