Introduction The world is changing and during the last years people are speeding up the process. Studies on how climate is changing, and how these changes differ along latitudes, are much more advanced than studies on how global change will affect plant community structure and how it relates to soil symbionts along different ecosystems. This is because changes that occur aboveground are easier to measure than those occurring belowground. Arbuscular mycorrhizal fungi (AMF) are belowground plant symbionts; they are a powerful link in the chain of transfers by which carbon (C) moves from the atmosphere to the plant and finally to soil sinks [1, 2]. They can potentially change C cycling rates by influencing plant growth and plant diversity [3]. Thus, any factor that changes mycorrhizal functions undoubtedly will affect primary productivity and soil C stock. Although it is widely acknowledged that AMF have multiple effects on terrestrial ecosystems, their relative contribution to ecosystem processes is unknown. As environmental conditions will change globally in the next few decades, probably at unprecedent rates, the behaviour of those AMF will play a substantial part in the response of ecosystems. Furthermore, because AMF represent an interface between the soil-plant system, their potential ability to regulate plant response to global change is one key reason why their responses need to be understood. We need to identify major obstacles that prevent us to

gain a full understanding where our knowledge is surprisingly poor. There is an obvious need to understand the impact that global change, especially increases in atmospheric carbon dioxide (CO2), will have on mycorrhizal symbioses. In this chapter we will focus on recent advances in our understanding of mycorrhizal association responses to environmental change, mainly to increases in atmospheric CO2.