NUL We reported here the concepts, applications and side effects of radiotherapy on the human tissue. The properties of x and γ rays, and their applications on the tumor tissues such as radiation therapy, gamma camera and gamma Knife are presented. Further, the physics of laser and its applications such as photo-coagulation, photo-vaporization, photo-chemical ablation, laser surgery, and laser in dentistry, laser in ophthalmology and laser in hemoglobin are also mentioned. Moreover, the side effects of those radiations on the normal tissues of the human body are discussed. On the other hand, the different methods of medical imaging such as radiography, computed tomography, fluoroscopy, magnetic resonance imaging (MRI) and nuclear magnetic imaging (NMI) are presented.