Organ growth in plants is controlled by both genetic factors and environmental inputs. Recent progress has been made in identifying genetic determinants of final organ size and in characterizing a pathway that may link organ growth with environmental conditions. Some identified growth regulatory factors act downstream of plant hormones, while others appear to be components of novel signaling pathways. Additional characterization of these proteins is needed before we can understand how growth-promoting and growth-restricting inputs are integrated to coordinate growth within a developing organ. Some parallels in the mechanisms used by plants and animals to regulate organ size are suggested by the identification of KLUH, a noncell-autonomous regulator of organ growth, and by similarities in the target of rapamycin (TOR)-signaling pathway.