A mycorrhiza is a symbiotic association between a fungus and the roots of a plant. The association is characterized by the movement of plant-produced carbon to the fungus and fungal-acquired nutrients to the plant. Mycorrhizas probably played a crucial role in the initial colonisation of land by plants. The establishment of the mycorrhiza involves specific developmental adaptations in both symbiotic partners. Mycorrhiza is divided into two main types: endomycorrhiza and ectomycorrhiza. There is also a transitional form, ectoendomycorrhiza. The most common and most important type of endomycorrhiza is arbuscular mycorrhiza. Establishment of arbuscular mycorrhiza starts by exchanging signals between fungi and plants. Signal exchange induces hyphal and root branching. When the fungus reaches the root surface, it forms appressoria, from which an infection hypha penetrates deep into the parenchyma cortex. Here, intercellular hyphae branch into root cells, where they differentiate into highly ramified arbuscules. Ericoid mycorrhiza, orchid mycorrhiza and ectomycorrhiza are also important types of mychorrizas. Given that mycorrhizas are nearly always beneficial to a plants growth and often health, their potential use to humans, in terms of agriculture and horticulture, could be immense. Despite this potential inoculation of crops is rarely seen and mycorrhizas are only introduced deliberately in a few industries. The main reasons are techniques of inoculum production and isolation, in which are still improving.