Radiation of different wavelengths can kill living organisms, although, the mechanism of interactions differs depending on their energies. Understanding the interaction of radiation with living cells is important to assess their harmful effects and also to identify their therapeutic potential. Temporally, this interaction can be broadly divided in three stages – physical, chemical and biological. While radiation can affect all the important macromolecules of the cells, particularly important is the damage to its genetic material, the DNA. The consequences of irradiation include- DNA damage, mutation, cross-linkages with other molecules, chromosomal aberrations and DNA repair leading to altered gene expression and/or cell death. Mutations in DNA can lead to heritable changes and is important for the induction of cancer. While some of these effects are through direct interaction of radiation with the target, radiation can interact with the surrounding environment to result in its indirect actions. The effects of radiation depend not only on the total dose but also on the dose rate, LET etc. and also on the cell types. However, action of radiation on organisms is not restricted to interactions with irradiated cells, i.e. target cells alone; it also exerts non-targeted effects on neighboring unexposed cells to produce productive responses; this is known as bystander effect. The bystander effects of ionizing radiations are well documented and contribute largely to the relapse of cancer and secondary tumors after radiotherapy. Irradiation of cells with non-ionizing Ultra-Violet light also exhibits bystander responses, but such responses are very distinct from that produced by ionizing radiations.