This book explains in an accessible way how the concept of the photon was introduced by Einstein who also helped discover many of its properties, and how it interacted with many physical phenomena while revolutionizing the world of physics. Replete with short biographies of the main contributors to the history, the book begins with the historical and scientific conditions that led to the quantization of the electromagnetic field. This upended previously held assumptions about the nature of radiation and led to the christening of the photon. The book then traces how this breakthrough inspired further investigation into the quantum world, as well as setting the stage for many dramatic advances in quantum science and technology over the decades that followed. For example, the dual wave-particle nature of the electromagnetic radiation was revealed by the photon, and extended to matter by Louis de Broglie; entanglement, which surfaced in discussions among Einstein, Bohr and Schroedinger, and is one of the most fundamental characteristic features of the quantum world, is realized beautifully by the photon and is the key to future technologies such as quantum computing and quantum teleportation. The book is essential reading for anyone with an interest in optics, quantum physics or the history of science. It is accessible to anyone familiar with elementary mathematics and basic quantum mechanics, and will be useful to students who will find within a gentle yet rigorous introduction to the photon’s properties.