Construction History is an area that has grown in the last twenty years across the world. This lecture looks at the questions it can ask and the role it can play, not only in understanding the past, but also informing future design for architectural engineers. It looks specifically at Christopher Wren and how he developed the designs for St Paul’s Cathedral in London. It starts with how the project demonstrates that designers in the past were often closer to architectural engineers than to architects. This leads to a discussion of the various approaches to construction history and its uses and how the survival of the original documents (in the form of contracts, letters, drawings and building accounts) allows us to trace processes in the past. It looks at the way the site at St Paul’s was organised and how the demolition of the previous building was carried out. It looks at the problems of finance and construction supervision. It then discusses the lessons from St Paul’s about geotechnics, particularly the necessity of monitoring, with a brief excursion into the issues of historic buildings and earthquake design. It then explores the construction of the masonry walls, the materials used, the problems of the weathering of walls and the necessity of sacrificial materials. The roofs are then discussed, both as examples of the architectural engineer at work and of the transfer of technology across cultures. The lecture will discuss the extraordinary staircases and their place in the history of staircase design, how the windows relate to modern structural glazing and how it uses an ingenious drainage system to ensure the structure is not damaged. Finally, it looks at the problems of designing, the doors, finishing the dome and deciding when a building is complete. In this way the lecture seeks to use St Paul’s to give a very brief overview of what construction history can bring not only to the understanding of one particular building but also to the modern world.