The leaf is a suitable subject with which to study plant morphogenesis because of its diversity of shape. Although mechanisms for leaf initiation and lateral morphogenesis have been suggested, the exact means for determining shape remain unclear. Many genes involved in those developmental processes have now been identified. Here, we summarize the early events in the genetic regulation of Arabidopsis leaf formation, including initiation, dorsoventrality, and the spatial and temporal control of cell proliferation and enlargement. We focus on recent progress within the model plant Arabidopsis, placing special emphasis on our own findings.