The modern movement of the 1920s supported a programme of social reform through rationalisation and industrialisation. Nowadays, economical building is based financial considerations as well as the limited availability of material and energy resources. “Building” is increasingly a practice of “repairing”. This represents a paradigm shift for the discipline of architecture, with designers reinventing themselves in different roles and relying on the latest technologies and age-old knowledge. Drawing on talks between academics and practicing architects, this issue revisits the concept of rationality in architecture and explores how this shift is being addressed in different contexts today.