Elevated atmospheric CO 2 concentrations (c a) will undoubtedly affect the metabolism of tropical forests worldwide; however, critical aspects of how tropical forests will respond remain largely unknown. Here, we review the current state of knowledge about physiological and ecological responses, with the aim of providing a framework that can help to guide future experimental research. Modelling studies have indicated that elevated c a can potentially stimulate photosynthesis more in the tropics than at higher latitudes, because suppression of photorespiration by elevated c a increases with temperature. However, canopy leaves in tropical forests could also potentially reach a high temperature threshold under elevated c a that will moderate the rise in photosynthesis. Belowground responses, including fine root production, nutrient foraging and soil organic matter processing, will be especially important to the integrated ecosystem response to elevated c a . Water use efficiency will increase as c a rises, potentially impacting upon soil moisture status and nutrient availability. Recruitment may be differentially altered for some functional groups, potentially decreasing ecosystem carbon storage. Whole-forest CO 2 enrichment experiments are urgently needed to test predictions of tropical forest functioning under elevated c a . Smaller scale experiments in the understorey and in gaps would also be informative, and could provide stepping stones towards stand-scale manipulations.