For most plants, leaves function as solar panels, equipped with photosynthetic cells that transform carbon dioxide and water into carbohydrates and oxygen. To fulfill this function, the initiation and shape of leaves are precisely regulated in each species. In addition, many plant organs are derived from leaves. For example, all floral organs are specialized leaves, as first proposed by von Goethe in 1790. Thus, leaf development forms the developmental basis of many other plant organs. In this chapter, we will first review the basic structure of leaves. Next, we will review the molecular pathways underlying leaf development. We will discuss (1) how a leaf is specified in the shoot meristem, (2) how the leaf attains its flattened shape and polarity, and (3) how the leaf cell proliferation is activated and then terminated. In addition, we will review variations of leaf shape, in particular, compound leaf formation. Special emphasis is given to the recent findings of phytohormonal and mechanical regulation of leaf shape patterning.