The study of organ size control is a discipline of developmental biology that is largely unexplored. Although the size of an organ or organism depends largely on cell numbers and cell size, studies have found that the simple deregulation of cell proliferation or cell growth does not necessarily lead to changes in organ size. Recent genetic screens in Drosophila suggest that mutations that do affect organ size can be classified into three broad categories on the basis of their underlying effects: patterning, proliferation, and growth. Overall, experimental data suggest that organ size might be regulated by a 'total mass checkpoint' mechanism which functions to link the regulation of cell size and cell proliferation. The mechanisms of organ size control could also be critical targets for evolutionary events or disease processes such as tumorigenesis.