Man-made causes of decay are complicated and have widespread implications in the conservation of historic buildings. So little has been done to prevent this type of decay that the present chapter can provide few answers to the questions it raises, because the problems are immense, involving economics and industrialization. What can be said is that unless the causes of decay of cultural property are properly analysed and the effects of each harmful agent to some extent quantified, there is a danger that the wrong priorities will be applied to protective measures. Much more measurement and evaluation of the effect of man-made causes of decay is needed. To obtain an approximate measure, comparative studies should be made of the rates of decay in different environments. The American practice of having Environmental Impact Statements is a step in the right direction. If decay were measured on significant buildings internationally, we could use them as laboratories of experience.