The colonization and deterioration of building materials from three Spanish cathedrals by lichens and bryophytes was investigated. The materials included sandstones, limestones, granites, tiles, flagstones, and mortars. The colonization of the different types of materials by lichens was induced, to a great extent, by the input of bird excrement, giving rise to a characteristic ornithocoprophilous flora. Communities of mosses grew preferentially on mortars with pronounced disintegrating effects due to penetration of rhizoids. Both lichens and bryophytes lead to mechanical and chemical damage to stonework. This represents a stage in the succession which gives rise to the later invasion by vascular plants, whose roots cause important deterioration in the structure of the buildings, mainly roofings, tiles, gutters, etc.