The objective of this research was to determine the impact of algal colonisation on natural stone masonry. Experimental work was carried out to determine the physical and chemical damage caused by micro-algae using reflected light microscopy, Scanning Electron Microscopy and Inductively Coupled Plasma - Atomic Emission Spectroscopy. Colonisation experiments were performed on individual mineral chips of quartz, calcite, dolomite, siderite, labradorite, orthoclase, a perthitic albite, muscovite and montmorillonite; as well as the Giffnock sandstone, a traditional building stone of the Glasgow area. Work was also carried out to determine the effect of algal colonisation on the absorption of water into a masonry surface. The research determined that algae create an alkaline environment in the areas they colonise. Algal mediated damage to the mineral substrates includes the dissolution and pitting of carbonate surfaces as well as the etching of plagioclase feldspar surfaces. Algal colonisation preferences were noted throughout the experiment with algae preferentially colonising kinks and steps in the topography of mineral surfaces as well as grain edges. Preferences were also seen in the colonisation of the Giffnock sandstone with micas showing heavy colonisation compared to other minerals in the lithology. Algal swelling and contraction cycles were examined in Environmental Scanning Electron Microscopy experiments and the impact that this physical swelling may have on the stone is modelled. Algal biofilms at the surface of the stone lead to an acceleration in the rate at which water enters the surface of the stone, this is important as water is the main weathering catalyst for masonry weathering. The findings of this project implicate algae in the weathering of natural stone masonry through enhanced mineral dissolution, mineral etching and pitting, patina formation, physical weathering through swelling cycles and the alteration of the surface physical properties in relation to water absorption.