This introductory essay highlights the key themes that appear in the five essays that make up the special issue: 'Complex Interior Spaces in London, 1850-1930', which focuses on street markets, railway stations, winter gardens and people's palaces, and a hospital. Those themes include complexity and multifunctionality; nodes and networks; modernity; materiality and spatiality; the public/private spheres; and user experience. The fact that the essays emanate from a design historical perspective places a new emphasis on the complex interiors of the buildings under review, and on the activities than went on in them, rather than on their architectural facades. While these building types were not unique to London, this introduction suggests that their size and scale were particular to that city.