This symposium is proposed approximately fifty years after the seminal work of Fraenkel (1959) placed the ecological function of plant secondary metabolites (PSMs) onto the agenda of modern ecology. Since then numerous ecological roles of PSM have been elucidated, including the mediation of interactions between pairs of species, such as between plants and their herbivores and pathogens, as well as acting against other plants. However, important recently emerging developments have been made in our understanding of the processes underlying the ecological roles of PSMs