Ecosystems worldwide are influenced by invasive species in their functioning in a multitude of ways. The success of invasive plants must be regarded as highly context-dependent and linked to a combination of both abiotic and biotic factors, and multiple mechanisms. In particular, biotic interactions of invasive species with their new environment may be the key driver for the successful spread into new areas. The author Cornelia Bäucker reports about the interaction of invasive plants with belowground organisms, especially arbuscular mycorrhizal (AM) fungi. In addition to a general introduction and a summary of the relevant results, this book consists of three studies, which are structured as journal articles. Two studies focus on the importance of the AM symbiosis for the successful spread of common ragweed in the new European range. Another study is dedicated to the phenomenon of heterocarpy of the non-native plant species gallant soldier in a soil feedback experiment. This book is intended for both national and international scientists, especially plant ecologists and mycorrhizal experts.