Plant organs grow to characteristic sizes and shapes that are dictated by the plant's genotype and the identity of the organ. Significant progress has been made in identifying and characterizing regulatory factors that promote organ growth, which act either on cell proliferation or on cell expansion. Their activity is antagonized by repressors of growth that limit organ size. Although the way in which that genes determine the identity of an organ modify its growth patterns is still unclear, initial links between growth regulators and patterning activities are being uncovered. As for the differences in organ size and shape between plant species, studies of natural variation are beginning to shed light on the underlying molecular changes.