Efficient communication between the pest-colonised and non-colonised organs is vital for the timely manifestation of defences that restrict systemic spread of pests. The vasculature provides an important conduit for translocation of signals that contribute to long-distance communication within a plant. Airborne signals also contribute to this process. In recent years, major advances have been made in identifying metabolites that are candidate systemic signals in plant defence against pathogens. Methyl salicylate, jasmonates, azelaic acid and a diterpenoid have been implicated as mobile signals associated with the activation of systemic acquired resistance (SAR), which confers enhanced resistance against a broad spectrum of pathogens. By contrast, auxins probably contribute to negative regulation of systemic defences.