Gravitational waves of different frequencies are generated by different sources. As examples, the late inspiral and merger of two neutron stars give high frequency (around 100 Hz) gravitational waves that can be detected by ground-based detectors. On the other hand, inspiral and mergers of super-massive black holes (extra-galactic) produce gravitational waves of nano-Hz frequency, which are beyond the reach of the ground or spaced-based human-made interferometric detectors. Interestingly, not only are gravitational waves created by individual mergers, there is a stochastic background of nano-Hz gravitational waves due to the superposition of many inspiraling supermassive black holes. It is possible to detect such low-frequency gravitational waves by finding their imprints on the electromagnetic waves emitted from the stars in our Galaxy by measuring the differential light travel times. However, every type of star is not suitable for this experiment. Pulsars, whose rotational periods are very stable and act as the most precise clocks, can be used for this purpose. Hence, there is an ongoing effort to detect nano-Hz gravitational waves by observing a number of pulsars. This effort is known as the International Pulsar Timing Array and member consortia are the Australian Parkes Pulsar Timing Array (PPTA), the North American Gravitational Wave Observatory (NANOGrav), the European Pulsar Timing Array (EPTA), the Indian Pulsar Timing Array (InPTA, an Indo-Japanese collaboration) and associate members are the Chinese Pulsar Timing Array (CPTA) and the South African MeerTime collaboration. Recently, these consortia notices the hint of a stochastic background of nano-Hz gravitational waves in their data. In the present talk, I will briefly describe why and how pulsars can be used to detect gravitational waves and some technical details of these consortia, their results, and the uniqueness of the Indian effort.