This chapter provides an anatomical and cytological description of vesicular-arbuscular mycorrhizae (VAM), as they are in angiosperm roots, for most of the available data concern plants from this phylum. It also provides a description and comparison of colonization processes occurring in bryophytes, pteridophytes and gymnosperms, which are traditionally said to form VAM infections. The morphology of the infective process is definitely very much like that found in the VAM associations in angiosperms. During their life cycle, ferns may display a fungal infection in their thalli, rhizomes, or roots. The development and spread of the extramatrical phase of VAM fungi differs greatly according to the type of soil, plants, and fungi. Vesicles are globose bodies caused by an intercalary or terminal swelling of a hypha of the VAM fungus. Vesicles found within roots can be intercellular or intracellular, differ in size, and may be found in both the inner and the outer layers of the cortical parenchyma.