Proton beam therapy has been developed for about the last ten years as one of the most reasonable treatment techniques in cancer therapy, based upon the beneficial features of proton beams. Because of the favorable clinical results obtained further progress in this form of treatment is expected. The fundamental and clinical features of this technique and also the future plans being studied at the moment are discussed here, based on the clinical results obtained mainly at the Particle Radiation Medical Science Center of Tsukuba University. The fundamental features of a proton beam are the possibility of selective irradiation of the tumor lesion and minimal irradiation of the surrounding normal tissues due to its fine dose distribution and favorable biological response which is similar to that of 60Co gamma rays. Due to these beneficial features of proton beams, tumor lesions have been successfully irradiated with sufficiently large doses, avoiding irradiation of important organs close to the lesions. Treatment results revealed sufficient control of lesions resistant to conventional radiotherapy, such as locally advanced lesions, lesions with large volume or those with low sensitivity. These results suggested that proton beam therapy could be applied to radical treatment of lesions over a much wider indication range, especially in deep-seated organs, compared with conventional radiotherapy. Investigations of proton beam treatment are to be further continued with the development of techniques to delives precisely controlled irradiation to lesions in moving organs and of a treatment unit installable in cancer hospitals.