Cell injury is defined as a variety of stresses a cell encounters as a result of changes in its internal and external environment. The cellular response to stress may vary and depends upon the type of cell and tissue involved and the extent and type of cell injury. A cell comprises cell membrane, nucleus, cytosol or the cytoplasm and cell organelles. All cells in the body constantly exchange information with each other to perform their functions properly. This process is accomplished in the cells by direct cell-to-cell contact, and by chemical agents, also called as molecular agents. The cells may be injured by genetic causes or by acquired causes. Cellular aging or senescence leads to impaired ability of the cells to undergo replication and repair, and ultimately lead to cell death culminating in death of the individual. Deficiency of oxygen or hypoxia results in failure to carry out the activities by the cells. Hypoxia is the most common cause of cell injury and is mainly caused by reduced supply of blood to cells due to interruption i. e. ischaemia. Killing of cells by ionizing radiation is the result of direct formation of hydroxyl radicals from radiolysis of water. These hydroxyl radicals damage the cell membrane as well as may interact with DNA of the target cell.