Leaves extend a large, porous surface to the environment to catch light and exchange gasses. The extension of the lamina is produced by the interaction of an upper (adaxial) and a lower (abaxial) domain in the developing leaf primordium. Recent studies have revealed that conserved genetic pathways, involving small regulatory RNAs and several distinct transcription factor families, have key roles in adaxial-abaxial patterning, suggesting candidate signals that convey positional information within the shoot to the newly initiated leaf. The interactions of the polarity pathways are distinguished by mutual antagonism and by redundancies. Analysis of these pathways in different model organisms reveals a surprising diversity in the genetic control of such a fundamental developmental process.