This paper discusses the economic and townscape effects of large-scale urban fires and the marked reduction of the problem in developed world cities during the nineteenth century. At that time a ‘fire gap’, or divergence between the increasing urban population and the falling absolute number of fires, demonstrably emerged. The paper outlines two processes - construction and rebuilding in less flammable materials, and increases in house lot size - which made for a more durable urban environment. This fortunate result, which in general owed little to the effects of urban planning and replanning, was largely a product of rising incomes. Examples are drawn from cities in Britain, North America, and Australia, and are contrasted with cities in the pre-modern Third World.