Abstract: The use of carbon ion beam therapy for tumor treatment offers unique physical and biological advantages, high efficiency, low toxicity, and a short treatment cycle. Therefore, it is considered to be the most ideal radiation therapy for cancer in the 21st century. However, construction and equipment operation costs for a carbon ion center are prohibitive. Currently, carbon ions are only used for clinical purposes in Japan, Germany, China, Italy, Austria, and a few other countries. In Japan, patients with cancer have been treated with carbon ions since 1994, with strong positive effects. To date, over 46,800 patients worldwide have undergone carbon ion radiotherapy (CIRT) for conditions such as malignant head and neck tumors as well as lung cancer, liver cancer, pancreatic cancer, and other cancer types. We conducted a review encompassing over 20 years of global clinical experience with CIRT to summarize treatment outcomes, recent developments, and the merits of CIRT as an emerging cancer treatment modality. We also compare its efficacy and toxicity with those of photon radiotherapy. These findings can serve as a useful reference in decision making and planning regarding new heavy ion centers to better serve patients with cancer.