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.

There is little information or evidence about major architectural theory in antiquity, until the 1st century BC, with the work of Vitruvius. This does not mean, however, that such works did not exist, given that many works never survived antiquity. Vitruvius was a Roman writer, architect, and engineer active in the 1st century BC. He was the most prominent architectural theorist in the Roman Empire known today, having written De architectura (known today as The Ten Books of Architecture), a treatise written in Latin and Greek on architecture, dedicated to the emperor Augustus. Probably written between 27 and 23 BC, it is the only major contemporary source on classical architecture to have survived. Divided into ten sections or "books", it covers almost every aspect of Roman architecture, from town planning, materials, decorations, temples, water supplies, etc. It rigorously defines the classical orders of architecture. It also proposes the three fundamental laws that architecture must obey, in order to be so considered: firmitas, utilitas, venustas, translated in the 17th century by Sir Henry Wotton into the English slogan firmness, commodity, and delight (meaning structural adequacy, functional adequacy, and beauty).

The rediscovery of Vitruvius' work in 1414 had a profound influence on architects of the Renaissance, adding archaeological underpinnings to the rise of the Renaissance style, which was already under way. Renaissance architects such as Brunelleschi and Leon Battista Alberti found in De architectura their rationale for raising their branch of knowledge to a scientific discipline.

Vastu Shastra (vāstu śāstra - literally "science of architecture" are texts on the traditional Indian system of architecture. These texts describe principles of design, layout, measurements, ground preparation, space arrangement, and spatial geometry. The designs aim to integrate architecture with nature, the relative functions of various parts of the structure, and ancient beliefs utilising geometric patterns (yantra), symmetry, and directional alignments.

Vastu Shastra are the textual part of Vastu Vidya - the broader knowledge about architecture and design theories from ancient India. Varahamihira's Brihat Samhita dated to about the sixth century CE is among the earliest known Indian texts with dedicated chapters with principles of architecture. For example, Chapter 53 of the Brihat Samhita is titled "On architecture", and there and elsewhere it discusses elements of Vastu Shastra such as "planning cities and buildings" and "house structures, orientation, storeys, building balconies" along with other topics. Other ancient Vastu Shastra works includes Manasara etc.

Following the tradition of Vastu Shastra, several scholars wrote architectural texts during medieval times which includes Manushyalaya Chandrika, dealing with domestic architecture authored by Thirumangalath Neelakanthan Musath, Samrangana Sutradhara written by Bhoja of Dhar, a poetic treatise on classical Indian architecture among others.

Throughout the Middle Ages, architectural knowledge was passed by transcription, word of mouth and technically in master builders' lodges. Due to the laborious nature of transcription, few examples of architectural theory were penned during this time. Most written works during this period were theological, and were transcriptions of the Bible. Since the architectural theories were on structures, fewer of them were transcribed. The Abbot Suger's Liber de rebus in administratione sua gestis was an architectural document that emerged with Gothic architecture. Another was Villard de Honnecourt's portfolio of drawings from about the 1230s.

In Song dynasty China, Li Jie published the Yingzao Fashi in 1103, which was an architectural treatise that codified elements of Chinese architecture.