Presence of radioactive energy was discovered more than 100 years ago. We are exposed to ubiquitous background radiation as well as anthropogenic sources of radiation in our day to day life. Exposure to ubiquitous background radiation has not increased significantly over the last 25 years. On the other hand, the amount of radiation exposure from medical sources has increased dramatically due to increased utilization of technology like computed tomography, nuclear medicine and interventional fluoroscopy. In this article, we describe the different kinds of radiation, sources of radiation and various measures of radiation exposure.