Brief description Gravitational waves are a prediction of Einstein's general theory of relativity. Despite many searches over several decades and much current effort, there has never been a direct detection of these waves. Such a detection would open up a window into presently unobservable phenomena such as orbiting black holes in the cores of distant galaxies. Pulsars are such precise clocks that the effect of a weak gravitational wave from such sources passing over the Earth may be detectable. The Parkes Pulsar Timing Array project aims to make precise timing measurements on 20 millisecond pulsars with observations every two weeks for five years. Modelling suggests that with the improved instrumentation currently being developed and this dataspan we will have enough sensitivity to detect gravitational waves. The data will also allow us to establish a pulsar timescale which should be more precise than the best terrestrial timescales based on atomic clocks.