In the developmental process of plant cells, cell fate is determined by expression of specific genes. Their expressions are directed by specific transcriptional regulators and positional signals that convey the locations of respective cells along various body axes, including proximal–distal, adaxial–abaxial and medial–lateral axes. Recently it was reported that some small regulatory RNAs work in a non-cell-autonomous fashion and provide neighboring cells with positional signals. Among such small RNAs, we will review unique biological features of microRNA 165 ⁄ 166 widely involved in plant development.