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The flattening of leaves results from the juxtaposition of upper (adaxial) and lower (abaxial) domains in the developing leaf primordium. The adaxial-abaxial axis reflects positional differences in the leaf relative to the meristem and is established by redundant genetic pathways that interpret this asymmetry through instructive, possibly non-cell autonomous, signals. Small RNAs have been found to play a crucial role in this process, and specify mutually antagonistic fates. Here, we review both classical and recently-discovered factors that contribute to leaf polarity, as well as the candidate positional signals that their existence implies