Plants maintain the ability to form lateral appendages throughout their life cycle and form leaves as the principal lateral appendages of the stem.Leaves initiate at the peripheral zone of the shoot apical meristem and then develop into flattened structures.In most plants,the leaf functions as a solar panel,where photosynthesis converts carbon dioxide and water into carbohydrates and oxygen.To produce structures that can optimally fulfill this function,plants precisely control the initiation,shape,and polarity of leaves.Moreover,leaf development is highly flexible but follows common themes with conserved regulatory mechanisms.Leaves may have evolved from lateral branches that are converted into determinate,flattened structures.Many other plant parts,such as floral organs,are considered specialized leaves,and thus leaf development underlies their morphogenesis.Here,we review recent advances in the understanding of how three-dimensional leaf forms are established.We focus on how genes,phytohormones,and mechanical properties modulate leaf development,and discuss these factors in the context of leaf initiation,polarity establishment and maintenance,leaf flattening,and intercalary growth.