Unlike their counterparts lichens and bryophytes, the role of epiphytic terrestrial algae as indicators of atmospheric pollution is not widely explored by biologists. This thesis investigates the relationship between the two factors using green algae due to their high degree of exposure to pollutants and their potential for rapid response to environmental change. We looked into how contemporary atmospheric pollutants influence the growth, abundance and diversity of epiphytic terrestrial algae in the UK. Approaches included surveying algal abundance along a local (150m) transect adjacent to a point source of ammonia pollution, and a similar survey along a much longer transect (50km) spanning urban and rural locations. Controlled application of nitrogen and sulphur gaseous pollutants was also undertaken, initially on a small scale using quadrats on tree trunks, and on a larger scale using the facility operated by the Centre for Ecology & Hydrology at Whim bog, south of Edinburgh. The response of selected algae to the same chemicals under laboratory conditions was also investigated. Through the five separate but related field and laboratory experiments, it is concluded that nitrogen (N) deposition is probably the most important factor controlling the growth of green epiphytic algae as opposed to other pollutants such as sulphur dioxide. Local high N deposition is markedly stimulatory to the growth and sustainability of Desmococcus olivaceus. It also contributes to the dominance of this and other nitrophilous algae and suppresses acidophyte species. Algae exposed to wet N deposition show greater species diversity compared to dry N deposition. In general Desmococcus spp. were much more tolerant of pollution than Trentepohlia spp. Long-term experiments at both Whim bog (Edinburgh, Scotland) and Silwood Park (Ascot, England) showed that a reduced form of N was more stimulatory to algal growth than oxidized N. In contrast, over short-term exposure periods, oxidized N in general was more beneficial to algal growth than the reduced form. The same contrasting result between short and long term exposure was also observed where bisulphite, as a proxy for sulphur dioxide, did not show any toxicity to algae under the short-term study but was damaging when exposed over a longer period.