In 1979 astronomers discovered the first gravitational lens, which produced two identical images of a distant quasar. In the intervening period a rich variety of gravitational lensing phenomena have been identified, with new examples emerging serendipitously and increasingly as a result of targetted searches. The detection and study of gravitational lenses is technically demanding but progress has been impressive and astronomers have reached a stage where studies of the frequency and properties of the different classes of gravitational lens should provide crucial information concerning the nature of the elusive dark matter and the evolution of galaxies early in the history of the universe.