Odors play important roles in the life of many animals including insects. The atmospheric gas CO2 presents an interesting and for humans relevant example, because CO2 is an attractive cue used by disease vectors such as mosquitoes to find their hosts for blood feeding. However, CO2 can elicit strong attraction but also avoidance behavior depending on the insect species. In contrast to mosquitoes, the vinegar fly Drosophila melanogaster is strongly repelled by even low levels of CO2 present for instance in an odor released by stressed flies. On the other hand, it is also produced by rotting fruit the main food source of these flies. CO2 is detected by a pair of dedicated 7-TM receptors, Gr21a and Gr63a, present on the olfactory organs of insects. Upon activation of CO2 sensory neurons, specific neural circuits are activated in the brain and trigger the avoidance behavior. We and others have recently shown that CO2 is processed by parallel neural pathways in a context-dependent manner. A hungry fly might use this parallel processing to overcome its aversion to CO2 in the context of foraging and feeding. Furthermore, our results suggest that innate CO2 behavior is modulated by the neurotransmitter dopamine. In my talk, I will present our latest efforts to understand how CO2 is modulated and processed in a context-dependent manner in the periphery and central brain.