Chemical defences against herbivores and infective microorganisms comprise preformed and elicitor-induced substances. Among others, phenolic compounds, sulphur-containing molecules and terpenoids can be active against biological stress agents. Metal ions themselves can act as elicitors or, according to the elemental defence hypothesis in metal hyperaccumulating plants, high tissue metal concentrations may themselves be effective against herbivores or pathogens. This paper provides a comprehensive overview on the different cross-points of signal transduction and metabolic pathways between metal ion toxicity and biotic stress factors. As examples two case studies are shown: one concerning glucosinolates in metal hyperaccumulator species of Thlaspi caerulescens, and a second about aluminium-induced alterations on benzoxazinoids in maize (Zea mays) root tips.