Gravitational lensing, the bending and distortion of light rays by intervening matter is a central prediction of Einstein's General Theory of Relativity, confirmed in 1919 on the occasion of a famous solar eclipse. However, Einstein was skeptical about its long term utility in cosmology. After remaining a dormant subject in astronomy for almost 70 years, gravitational lensing is now one of the most active and prominent research areas yielding important results on the nature and distribution of dark matter and the properties of the mysterious dark energy that causes the cosmic expansion to accelerate. Astronomers also use gravitational lensing to magnify otherwise inaccessible, distant objects bringing into view the earliest objects that shone when the Universe was in its infancy.