The characteristic size of plant seeds and organs is coordinately determined by cell proliferation and cell expansion.However,the genetic and molecular mechanisms that set the final size of seeds and organs are largely unknown and how plant organs know when to stop growing is a fascinating and unresolved problem in developmental biology.We have recently identified several seed and organ size regulators that are involved in ubiquitin-related activities in Arabidopsis.We will discuss the role of the ubiquitin pathway in seed and organ size control.