Plant organs grow to highly reproducible sizes that are determined by the plant's genotype and by the identity of the organ. The strong heritability of size differences indicates that organ size is under tight genetic control. The overall increase in size of plant organs is driven by two distinct processes: cell proliferation with the concomitant generation of cytoplasmic mass, and cell expansion due to water uptake into the central vacuole. Molecular genetic analysis has identified a number of genes that influence final organ size by acting on either of these two processes in a promoting or repressing manner. The differences in the sizes and shapes of the various organ types within individuals as well as the size differences among species result from the modification of growth patterns by factors controlling organ identity and by evolutionary change. Genetic analysis in model species is beginning to shed light on how these growth patterns are being controlled and modified. Together, these studies are unraveling how plant organs decide for or against further growth and suggest approaches for manipulating biomass accumulation in plants.