The process of cell competition results in the elimination of cells that are viable but “less fit” than surrounding cells. Given the highly heterogeneous nature of our tissues, it seems increasingly likely that cells are engaged in a “survival of the fittest” battle throughout life. The process has myriad positive roles in the organism: it selects against mutant cells in developing tissues, prevents the propagation of oncogenic cells, and eliminates damaged cells during ageing. However, “super-fit” cancer cells can exploit cell competition mechanisms to expand and spread. Here, we review the regulation, roles and risks of cell competition in organism development, ageing and disease.