Mycorrhizas are widespread symbiotic associations involving soil fungi and the roots of most land plants. They have been of primary importance in the evolution of land plants (Harley and Smith 1983; Simon et al. 1993) and are a key component of plant and fungal communities (Allen 1991; Chap. 11, Vol. V, Part B). Mycorrhizal symbioses have a beneficial impact on plant growth in natural and agroforestry ecosystems and allow the completion of the fungal life cycle (Harley and Smith 1983; Read 1991, 1992). The formation of the symbiosis requires several days and includes major changes in cellular and tissue morphology as well as in the biochemistry and physiology of the partners. Numerous recent reviews have covered the physiological and biochemical aspects of endoand ectomycorrhiza in detail (Smith and Gianinazzi-Pearson 1988; Jakobsen 1991; Söderström 1992; Martin and Botton 1993; Botton and Chalot 1995; Hampp and Schaeffer 1995). Alterations of the host plant and the fungal cell surface have been reviewed recently (Bonfante-Fasolo 1988; Bonfante and Perotto 1992; Bonfante 1994). Because mycorrhiza development involves the formation of specific multicellular structures, it incorporates the main features of developmental processes common in all organisms, i.e., temporal and spatial changes in cellular and tissular differentiation.