Léon Krier CVO is a Luxembourgish architect, architectural theorist, and urban planner, a prominent critic of modernist architecture and advocate of New Classical architecture and New Urbanism. Krier combines an international architecture and planning practice with writing and teaching. He is well known for his master plan for Poundbury, in Dorset, England. He is the younger brother of architect Rob Krier.

Architectural theory is the act of thinking, discussing, and writing about architecture. Architectural theory is taught in all architecture schools and is practiced by the world's leading architects. Some forms that architecture theory takes are the lecture or dialogue, the treatise or book, and the paper project or competition entry. Architectural theory is often didactic, and theorists tend to stay close to or work from within schools. It has existed in some form since antiquity, and as publishing became more common, architectural theory gained an increased richness. Books, magazines, and journals published an unprecedented number of works by architects and critics in the 20th century. As a result, styles and movements formed and dissolved much more quickly than the relatively enduring modes in earlier history. It is to be expected that the use of the internet will further the discourse on architecture in the 21st century.