The constants of nature are the numbers that define the essence of the Universe. They tell us how strong its forces are, and what its fundamental laws can do: the strength of gravity, of magnetism, the speed of light, and the masses of the smallest particles of matter. They encode the deepest secrets of the Universe and express at once our greatest knowledge and our greatest ignorance about the cosmos. Their existence has taught us the profound truth that Nature abounds with unseen regularities. Yet, while we have become skilled at measuring the values of these constants, our frustrating inability to explain or predict their values shows how much we still have to learn about the inner workings of the Universe. What is the ultimate status of these constants of Nature? Are they truly constant? Could life have evolved and persisted if they were even slightly different? And are there other Universes where they are different? These are some of the issues that this book grapples with. It looks back to the discoveries of the first constants of Nature and the impact they had on scientists like Einstein. This book also tells the story of a tantalising new development in astronomy. For the first time astronomical observations are suggesting that some of the constants of Nature were different when the Universe was younger. So are our laws of Nature slowly changing? Is anything about our Universe immune from the ravages of time? Are there any constants of Nature at all?