Bystander effects describe the effects of extracellular mediators from irradiated cells on neighbouring non-irradiated cells resulting in radiation-induced effects in unirradiated cells. Although the underlying mechanisms are largely unknown, it is widely recognised that two types of cellular communication (i.e. via gap junctions and/or release of molecular messengers into the extracellular environment) play an essential role. Additionally, the effects can be significantly modulated by parameters such as cell type, cell-cycle stage and cell density. Some of the common bystander effects or biological end points which are evidenced after low-dose irradiation are: chromosomal instability, cell killing and delayed cell death, mutagenesis, micronucleus formation, gene and protein expression changes. Through these end points it is likely that bystander effects can be both detrimental and beneficial. By increasing mutation levels of cells bystander effects increase the likelihood of genetic defects and in turn cancer. On the other hand by removing damaged cells from the population and preventing the growth of cancer cells, bystander effects are beneficial.Radiation hormesis is a term used to relate the beneficial effects of small doses of radiation on living cells, whether plant, animal or human. Experiments on bacteria, plants and animals have demonstrated that several biological mechanisms are stimulated by low dose radiation, such as: protein synthesis, gene activation, detoxication of free radicals and stimulation of the immune system. These mechanisms were also observed in humans.The present review paper is a compilation of the most recent data on bystander effects and the possible implications of cellular response to radiation on cell growth and development.