The study of animal organ size determination has provided evidence of the existence of organ‐intrinsic mechanisms that ‘sense’ and adjust organ growth. Cell competition, a form of cell interaction that equalizes cell population growth, has been proposed to play a role in organ size regulation. Cell competition involves a cell‐context dependent response triggered by perceived differences in cell growth and/or proliferation rates, resulting in apoptosis in growth‐disadvantaged cells and compensatory expansion of the more ‘fit’ cells. The mechanisms that allow cells to compare growth are not yet understood, but a number of genes and pathways have been implicated in cell competition. These include Myc, the members of the Hippo, JAK/STAT and WNT signaling pathways, and the Dlg/Lgl/Scrib and the Crb/Std/PatJ membrane protein complexes. Cell competition was initially characterized in the Drosophila imaginal disc, but several recent studies have shown that cell competition occurs in mouse embryonic stem cells and in the embryonic epiblast, where it plays a role in the regulation of early embryo size. In addition, competition‐like behavior has been described in the adult mouse liver and the hematopoietic stem cell compartment. These data indicate that cell competition plays a more universal role in organ size regulation. In addition, as some authors have suggested that similar types of competitive behavior may operate in during tumorigenesis, there may be additional practical reasons for understanding this fundamental process of intercellular communication. WIREs Dev Biol 2014, 3:419–427. doi: 10.1002/wdev.148This article is categorized under: Early Embryonic Development > Development to the Basic Body Plan Plant Development > Cell Growth and Differentiation Comparative Development and Evolution > Organ System Comparisons Between Species