The fourth session of the Construction History series will examine the various dimensions in which the threat of fire affected skyscraper development. Claims of “fireproof building” were regularly disproved, often in cataclysmic fashion. Iron promised improvements over timber, but Chicago’s Great Fire in 1871 revealed its vulnerability to collapse. Brick remained the only truly fireproof material, but owners and designers remained frustrated by its weight and inefficiency. The advent of lightweight terra cotta allowed architects to combine ceramic’s resistance to fire with iron’s efficient strength, leading to hybrid structures that allowed the safe exploitation of the skeletal frame. Fire also reshaped building codes, but new regulations reflected the competing desires of owners, tenants, architects, and skilled tradespersons that, in turn, influenced skyscraper massing and composition. Differing approaches in New York and Chicago forged subtly different solutions.