Cellular communication is at the heart of animal development, and guides the specification of cell fates, the movement of cells within and between tissues, and the coordinated arrangement of different body parts. During organ and tissue growth, cell–cell communication plays a critical role in decisions that determine whether cells survive to contribute to the organism. In this review, we discuss recent insights into cell competition, a social cellular phenomenon that selects the fittest cells in a tissue, and as such potentially contributes to the regulation of its growth and final size. The field of cell competition has seen a huge explosion in its study in the last several years, facilitated by the increasingly sophisticated genetic and molecular technology available in Drosophila and driven by its relevance to stem cell biology and human cancer. Developmental Dynamics 241:831–841, 2012. © 2012 Wiley Periodicals, Inc.