At the end of the eighteenth century, iron entered building at a large enough scale for it to be considered the first new material since pre-history. By the 1850s, a comprehensive set of ideas for the use of metals in construction had been developed, invented, debated and tested for all parts of habitable structures. This study proposes that iron came to building along two interconnected but distinct paths. One way was through processes of imitation and substitution dependant on lessons from other technologies alongside gradual evolution and refinement – approaches familiar to designers. The other route involved unprecedented innovations arising from original new ideas – the path of inventors. Investigating these overlapping routes that inaugurated a new era of building using iron, is a central theme in reviewing the developments, inventions and discoveries that were translated into optimistic proposals and practical solutions. The work calls upon original manuscripts and drawings from this early period as well as published material from disparate sources before specialist periodicals covering issues related to building emerged. A systematic analysis of British patents records up until the mid 1850s and accounts of new developments in the contemporary press, form a major and original part of the investigation. Analysis of surviving structures and reports of accidents and failures in structures using new techniques of fabrication and building will help contextualize the paths that lead to the implementation of new ideas. Acceptance of iron as a legitimate material for building went through phases of early unqualified optimism to guarded scepticism as a result of unanticipated disasters. In the main, builders, unfettered by aesthetic inhibitions, were opportunistic and pragmatic. They learnt to absorb the new material where it clearly offered advantages. Architects, particularly in Britain, went in the opposite direction, becoming progressively more coy at embracing the possibilities offered by iron. The research looks at how, despite the existence of a comprehensive vocabulary of iron construction that had been developed, tried and tested by the mid 1850s, architects, in Britain at least, were reluctant to embrace visible iron, while being open to its use performing essential duties behind finishes, hidden from view. These conservative attitudes in the country that initiated many early ideas for the use of iron in building will be contrasted with divergent attitudes in different contexts in Europe, the USA and further afield.