Proton beam therapy is an emerging technology with promise of revolutionizing the treatment of cancer. While nearly half of all patients diagnosed with cancer in the US receive radiation therapy, the majority is delivered via electron accelerators, where photons are used to irradiate cancerous tissue. Because of the physical properties of photon beams, photons may deposit energy along their entire path length through the body. On the other hand, a proton beam directed at a tumor travels in a straight trajectory towards its target, gives off most of its energy at a defined depth called the Bragg peak, and then stops. While photons often deposit more energy within the healthy tissues of the body than within the cancer itself, protons can deposit most of their cancer-killing energy within the area of the tumor. As a result, in the properly selected patients, proton beam therapy has the ability to improve cure rates by increasing the dose delivered to the tumor and simultaneously reduce side-effects by decreasing the dose to surrounding tissue. The benefits of proton beam therapy in delivering a lethal hit to the target while sparing surrounding normal tissues from radiation are becoming applicable to an increasing number of patients and a growing list of conditions. In this book, the author will guide the reader through existing evidence supporting proton beam therapy for pediatric cancer, prostate cancer, lung cancer, brain tumors, spinal tumors, and several other conditions. The book will discuss which conditions are suitable for treatment with proton beam therapy, how the treatment is delivered, and the current data supporting its use.