Ecologists have long been interested in the interplay between herbivore offense and plant defense. Chemical plant defenses have received much attention, but in 1994 it was suggested that unusually elevated levels of some elements (often associated with plants called hyperaccumulators of those elements) could function defensively. These “elemental defenses” differ from organic (secondary) plant chemicals because they are obtained from the soil and cannot be degraded by herbivores due to their elemental nature. A recent review (Boyd 2007; Plant and Soil 293: 153-76) concluded that defensive effects have been shown for five hyperaccumulated elements (As, Cd, Ni, Se, and Zn). Other elements hyperaccumulated by plants have yet to be experimentally tested, but it is clear that “elemental defenses” are another pathway by which plants chemically defend themselves against natural enemies. Recognition of elemental plant defenses by ecologists is growing (e.g., the 2002 edited book chapter on plant-insect interactions by Strauss & Zangerl in Herrera & Pellmyr’s book on plant-animal interactions; Schowalter’s 2006 text on insect ecology), but elemental defenses remain a relatively unrecognized and unexplored aspect of plant defense. The goal of this session is to bring together contributors from several labs that are studying elemental defenses to illustrate the exciting ecological questions currently being investigated. Talks in this session will include studies involving most of the elements studied to date: speakers will talk about As, Ni, Se, and Zn. We will hear evidence that some elements protect plants from natural enemies (Boyd, Palomino, Pilon-Smits, Quinn, Rathinasabapathi) but also that Zn might not (Noret), leading to discussion of how plant defense is defined and how it should be approached experimentally. Some speakers (Pilon-Smits, Quinn) will be able to include evolutionary aspects of elemental defenses, focusing mainly on the selective value of defense to a plant, but including the evolution of Se tolerance in an herbivore (Pilon-Smits). We will also see that there are community- and ecosystem-level processes influenced by these defenses (Boyd, Trumble, Butler) and so large-scale ecological phenomena can be impacted by elemental defenses. Finally, two speakers (Pilon-Smits, Rathinasabapathi) will address plant physiological processes that underlie hyperaccumulation of two elements (Se and As, respectively) and thus give insight into evolution of these traits at a molecular level.