In the conservation field of buildings, historical monuments and subterranean environments, very few studies highlight the importance to find alternative methods to chemical products against algal biofilm proliferation. Indeed, biofouling by microorganisms is a well-known biodeterioration phenomenon. Algae are considered as pioneers in the biodeterioration of stone, and that's why they are subject to a special interest in this chapter. Conservators of monuments or art works used many chemical applications to prevent or to control the algal proliferation on stone. But, due to the changes in European standards on their use, the chemical products must be replaced by environmentally friendly processes. The ultraviolet irradiation appeared to be a suitable process to control algal proliferation in these environments. Firstly in this chapter, we describe the problematic of algal biofilms (proliferation and biodeterioration) on different substrata and also the effects of applied chemical treatments. Secondly, we define and characterize the UV light and its effects on living organisms. Finally, we present the experimental design and the obtained results on algae harvested in caves at the cellular and molecular scales (pigment contents, photosynthetic activity, DNA damages...) and at the biofilm scale (proliferation measurements). This study shows that the UV treatment can be used as a conservation tool to eradicate algal biofilms which proliferate in buildings and monuments.