During the past ten years, significant progress has been made in understanding the basic mechanisms of the development of multicellular organisms. Genetic analysis of the development ofCaenorhabditis elegansandDrosophilahas unearthed a fruitful number of genes involved in establishing the basic body plan, patterning of limbs, specification of cell fate and regulation of programmed cell death. The genes involved in these developmental processes have been conserved throughout evolution and homologous genes are involved in the patterning of insect and human limbs. Despite these important discoveries, we have learned astonishingly little about one of the most obvious distinctions between animals: their difference in body size. The mass of the smallest mammal, the bumble–bee bat, is 2g while that of the largest mammal, the blue whale, is 150t or 150 million grams. Remarkably, even though they are in the same class, body size can vary up to 75–million–fold. Furthermore, this body growth can be finite in the case of most vertebrates or it can occur continuously throughout life, as for trees, molluscs and large crustaceans. Currently, we know comparatively little about the genetic control of body size. In this article we will review recent evidence from vertebrates and particularly from Drosophila that implicates insulin/insulin–like growth factor–I and other growth pathways in the control of cell, organ and body size.