A key feature of RNA interference is its ability to spread from cell to cell. Such non-cell-autonomous gene silencing has been characterized extensively in both plants and animals, but the identity of the mobile silencing signal has remained elusive. Several recent studies now shed light on the identity of this signal in plants, and indicate that small RNA molecules-from short-interfering RNAs to microRNAs-are capable of moving between cells and through the vasculature. The movement of small, 21-24-nucleotide RNA species has implications for biological processes ranging from developmental patterning and stress responses to epigenetic inheritance.