The harmful effects of ionizing radiation are very well documented. We cannot completely avoid the harmful effects of radiation however, while working in radiation area we can minimize the amount of radiation received by taking proper safety measure. The purpose of this paper is to review the various biological effects of radiation on human beings. Health care professional who uses ionizing radiation are concerned about the possible acute and chronic effects of occupational radiation exposure. One should have a clear understanding of biological effects of radiation so that proper care should be taken to avoid unnecessary exposure. The biological effect of ionizing radiation is a consequence of the energy transfer by ionization and excitation to body cells. Therefore, there is need to know the various biological effects of radiation on human beings so that we can use radiation safely. Natural populations have always been exposed to background levels of ionizing radiation. However, with the event of the nuclear age, studies about the effects of higher-than-background levels of ionizing radiation on individuals or populations of organisms became important. Background ionizing radiation arises from various manmade and natural sources present in the environment. We may get radiation from medical procedures, consumer products, industrial radiation sources, research activities, etc. Radiation professionals known as radiation worker may get exposure from radiation sources used by them for various applications3. Most common radiation exposure to the professionals is from various medical procedures used in Radiology, nuclear medicine, radiotherapy, etc.. Worldwide, the mean effective dose for medical workers was 1.6 mSv, and for interventional radiology was 3.0 mSv. In the United States, the mean annual effective dose during 2011 for physicians involved in fluoroscopically was 1.6 mSv. The harmful effects of radiation are very well known to us and while working in radiation area we can only minimize the amount of radiation received but we cannot avoid it completely 5,6. Therefore, there is need to know the various biological effects of radiation on human beings so that we can use radiation safely. Whenever ionizing radiation falls on human body, it produces ionization and excitation in the tissues and impairs the normal function of the cells. Thus human body will be subjected to biological damage and severity of this damage depends upon various factors mainly, nature and energy of the radiation, total dose & dose rate, the extent and part of the body exposed, age of the person exposed to radiation, radiation sensitivity of the organ exposed, etc. The interaction of ionizing radiation with human body could arise either from external radioactive sources or internal contamination leading to biological effects which may later show up as clinical symptoms. Radiobiological data have been derived mostly from micro-organism, cultured mammalian cells and whole animal systems. Human data is derived from the follow-up of the (a) survivors of atomic bomb explosions in Hiroshima and Nagasaki, (b) inhabitants of Marshall islands, who were exposed to fallout from thermonuclear devices, (c) uranium miners, (d) radium dial painters, (e) pioneer X-ray technicians and radiologists, (f) patients exposed to radiation for medical reasons; and (g) victims of nuclear accidents. A careful analysis of these data has yielded reasonable quantitative estimate of biological effects of radiation in man