As buildings age biological and non biological soiling accumulates on their facades. Soiling changes the visual appearance of buildings. This thesis investigates the aesthetic and perceptual changes which take place as a result of the accumulation of soiling. A series of experiments and surveys were conducted to investigate the effects soiling had on aesthetic and perceptual judgements of buildings. A multiple sorting procedure using photographs of buildings, revealed that both an aesthetic evaluation and soiling levels were important ways in which the buildings were conceptualised. A second study comparing photographs of architecturally similar buildings before and after stonecleaning had taken place, revealed large shifts in the evaluation of buildings following cleaning as measured by semantic differentials. Changes in evaluation were found to be dependent on the nature and outcome of the cleaning process. Buildings were also consistently perceived to be younger following cleaning. Surveys amongst residents of cities which had undergone major stonecleaning programmes revealed an awareness of this activity in line with theories of urban perception. Attitudes towards stonecleaning programmes was found to be generally favourable. A survey conducted amongst architects showed the complex range of aesthetic and perceptual effects which soiling and cleaning has on buildings. The surveys conducted amongst both the general public and architects revealed that while cleaned buildings were generally seen to be aesthetically more pleasing than heavily soiled ones, there were some situations where soiling could enhance the aesthetic appearance of buildings. A further study involving ratings of buildings which varied in terms of soiling was therefore conducted which further clarified the role of soiling in aesthetic judgements. Drawing on research in experimental and environmental aesthetics, as well as data from the reported experiments and surveys a model is proposed which relates soiling level to facade complexity and aesthetic evaluation.