In 1915, Albert Einstein presented his masterwork to the Prussian Academy of Sciences - a theory of gravity, matter, space and time: the General Theory of Relativity. Einstein himself said it was 'the most valuable theory of my life', and 'of incomparable beauty'. It describes the evolution of the Universe, black holes, the behaviour of orbiting neutron stars, gravitational lensing, and why clocks run slower on the surface of the Earth than in space. It even suggests the possibility of time travel. And yet we think instead of 1905, the year of Einstein's Special Theory of Relativity and his equation E=mc2, as his annus mirabilis, even though the Special Theory is less far-reaching. In this book, John and Mary Gribbin put Einstein's astonishing breakthrough in the context of his life and work, and make it clear why his greatest year was indeed 1915.