Cell competition is a cell fitness-sensing mechanism conserved from insects to mammals that eliminates
those cells that, although viable, are less fit than their neighbors. An important implication of cell competition
is that cellular fitness is not only a cell-intrinsic property but is also determined relative to the fitness of
neighboring cells: a cell that is of suboptimal fitness in one context may be ‘‘super-fit’’ in the context of a
different cell population. Here we discuss the mechanisms by which cell competition measures and communicates
cell fitness levels and the implications of this mechanism for development, regeneration, and tumor
progression