The RNAi pathway is present in almost all eukaryotes and relies on small RNAs directing sequence specific RNA degradation within the cell. In plants, small RNA directed pathways mediate viral defense, development, epigenetics, and responses to the environment. Our work has focused on the mechanisms that generate the sRNAs and the processes that they govern. Most recently we have been investigating the intercellular mobility of the sRNAs, the perception of viral sRNA and dsRNA, and the transitivity process that discriminates between self and non-self genes. We have identified host components that are intracellular sensors of viral invasion and this has helped us to develop a "starburst" model of defence. We have generated a mutant screen designed to identify components that regulate the mobility of sRNAs and a screen for components of the pathway that enables a plant to discriminate between artificial transgenes and inherited endogenous genes. Both of these screens have identified unexpected players: enzymes regulating the size of intercellular channels are important in siRNA spread and different members of the transcription termination complex are important in transitivity. A partially integrated model for the processes of perception, movement and epigenetic modification from sRNA-directed processes will be presented.