The medullary structure of 24 Umbilicariaceae species was studied by light microscopy. The objective was to investigate the variability of the medulla of these lichens and to assess the possible implications for thallus ecophysiology and the taxonomic relevance of the anatomy of this layer. Several methods of embedding, sectioning and staining together with the comparison of radial with transverse sections showed important differences in the degree of cohesion of hyphae and their spatial orientation within the medulla. Other important characteristics were the existence of more than one structurally homogeneous layer within the medulla, the frequency of hyphal branching, the relative thickness of the cell wall and lumen and the abundance of gelatinous matter in the intercellular spaces. The medullae studied were grouped into seven main types. The constancy of the types of medulla for certain groups of species makes it a useful secondary character to be considered in the taxonomy of the Umbilicariaceae. The important role of the structure of the medulla in the water relations of the Umbilicariaceae is discussed and a general relationship between medulla type and hydration strategy is suggested