How do the many millions of single cells that make up the body interact to ensure the integrity and smooth functioning of the “individual”? Though seemingly serene from the outside, the world inside is far from it. It is, in fact, teeming with interactions that at times follow a peaceful path of cooperation and at times one of competition and death. Like humans, single cells grow old and their performance decays through use and damage. Just as we are able to detect fitness decay in our human colleagues by looking at the graying of their hair, or the wrinkles in their faces, so are cells able to recognise decay in their neighbours. Once cells recognise an unfit neighbour they bring about its death and often replace it with a younger, healthier cell. Despite the crucial importance of this fundamental process and its implication into several broader fields, such as development, ageing, regeneration and cancer, the molecular mechanisms that underlie it are only starting to be uncovered.