A major contribution to our understanding of the basic laws of the universe -- from the author of The Book of Nothing. The constants of nature are the fundamental laws of physics that apply throughout the universe: gravity, velocity of light, electromagnetism and quantum mechanics. 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 have still to learn about inner workings of the universe. What is the ultimate status of these constants of nature? Are they truly constant? And are there other universes where they are different?