Leaves, the plant organs responsible for capturing and converting most of the 170 billion metric tons of carbon fixed globally each year, can be broadly grouped into two morphological categories: simple and compound. Although simple-leaved species such as corn and Arabidopsis have traditionally been favored model systems for studying leaf development, recent years have seen an increase in genetic and molecular studies of compound leaf development. Two compound-leaved species in particular have emerged as model systems: tomato and pea. A variety of mutations which alter leaf morphology in these species have been described, and analyses of these mutations have allowed the construction of testable models of leaf development. Also, the knotted-like homeobox (KNOX) genes, which were originally discovered as regulators of meristem function, now appear to have a role in compound leaf development. In addition to the recent genetic and molecular analyses of tomato and pea, insight into the nature of compound leaf development may be gained through the study of (a) heteroblasty and heterophylly, phenomena in which a range of leaf forms can be produced by a single shoot, and (b) the evolutionary origins of compound leaves.