The ecological importance of the extramatrical (EM) mycelia of mycorrhizal fungi is immense. To list but a few key functions; they are essential for nutrient uptake, growth and survival of most land plants. They are important for plant-plant interactions and for food webs in the soil. The production of the EM mycelium of ectomycorrhizas in forests might be in the same range as that of the fine roots. Thus the amount of carbon invested in the construction, operation and maintenance of this system is potentially very large. The importance of the EM mycelium in carbon cycling in ecosystems has only recently been recognised by a broader scientific community. However, currently our knowledge is limited of the basic parameters needed to calculate the extent of the EM mycelium in C dynamics such as variation in mycelial production, standing biomass and thus turnover rates, as well as its importance for the formation of stable soil carbon. Further, we also know very little about the regulating mechanisms behind such variations. In this paper we will first make a brief review of the current knowledge of the EM mycelium in C soil dynamics and identify important gaps in this knowledge. We will then present the methods currently available to estimate mycelial production and standing biomass, and subsequently turnover, and discuss their strengths and weaknesses.