This simple question is in fact one of the deepest, most long-standing problems in physics. None of the known laws of the universe can fully explain it. In The Janus Point, Julian Barbour presents a bold new thesis and a possible solution, with radical implications for our understanding of the Big Bang and the nature of time itself. His argument rests on two vital insights. The first is that the most common explanation for time - entropy - is flawed: firstly, because we have no way of explaining how the concentration of energy that would allow the Big Bang to take place came about, and secondly because none of our understanding of entropy takes into account the fact that the universe is infinitely expanding. In addition, our universe is actually becoming ever more complex and ordered as it expands, not less so. The second is a phenomenon which Barbour labels 'The Janus Point': any system of particles in motion will pass through a single moment of smallest size, never to be repeated. Combining these two observations, Barbour argues that the universe, and therefore time itself, may not have begun at the Big Bang, but rather at The Janus Point, thus solving the conundrum of entropy. Monumental in vision and scope, The Janus Point offers a ground-breaking challenge to our understanding of the universe and a brilliant solution of breath-taking elegance and import to this most fundamental of problems.