1.618033989... is a magic number. Its magic may not be as obvious as the most famous irrational, pi, nor as familiar as e (both of which are also transcendental), but its connection to the Fibonacci series (1, 1, 2, 3, 5, 8, ..., in which each element is the sum of the two previous) is a both intimate and surprising, and its role in the spiral of mollusc shells, inscribed pentagons, pineapple segments, fir cones, and the arrangement of seeds in a sunflower provides remarkable evidence that when nature speaks, she does so in the language of mathematics. Astrophysicist Mario Livio, who also wrote "The Equation That Couldn't Be Solved" (about group theory), takes a tempered approach to his subject. Claims have frequently been made that phi was the design principle of the Egyptian pyramids, the Parthenon, the works of Leonardo da Vinci, and in many other artistic creations. The evidence for most of them is weak - often based on a proportion that, when measured in a certain way, comes close to phi or its reciprocal. Livio is rightfully skeptical of most of these claims, but he also gives the Golden Ratio credit for works in which it is clearly implicated. This book provides a wonderful connection from science, art, music, and architecture to geometry and mathematics.