Modern architectural theory was a product of the encounter between the classical tradition, formed on Greek and Roman ideas in the Italian Renaissance, and the subjective aesthetics of eighteenth century Britain. Resulting ideas and buildings of the early nineteenth century were the precursors of the architecture and theories of the 1920's and 30's. The development of classical architectural theory is studied in the writings of the philosophers and architects who contributed its leading ideas. The relative importance of the two basic themes, 'reason' and 'experience', was established by Plato and Aristotle. The consequences of their ideas and priorities for architecture can be seen in the treatises of Vitruvius and Alberti. 'Reason' as the way to absolute beauty became the cornerstone of Renaissance theory, while 'experience', subjective and therefore relative, was rejected as leading only to inconsistent and erroneous opinion. In the seventeenth century the critical heritage of the Greeks bore new fruit in the rationalism of Descartes and his successors, and particularly the British 'empirical' philosophers of the eighteenth century. Criticism of classical theory by Perrault, Cordemoy and Laugier in France led to a more rigorous 'rational' theory, still essentially classical in its emphasis on the absolute qualities of building forms. In Britain rational analysis of subjective 'experience' led to a comprehensive aesthetic theory based on the association of ideas and most expounded by Archibald Alison in 1790. Further consequences of the rationalism of this period were the revolutionary economic and political changes which shook France and Britain and had far-reaching consequences for architecture. Most advanced in Britain, these changes together with the new aesthetic theories had their most direct architectural effects on the design of smaller houses. The many books of cottage and villa designs published around 1800 record both the architecture of the period and the ideas on which it was based. Particular attention is given to the writings of J.C. Loudon as the most comprehensive exposition of advanced pre-Victorian theory. The development of architectural ideas in Victorian and modern times shows both continuity with earlier streams of thought, and significant changes in particular as the failure of traditional theory to cope with social change became critical. The writings of Pugin, Ruskin, Fergusson, and Morris reveal the scope of Victorian theory and its contribution to modern thought. The transition from Victorian to modern architecture is shown in a comparison of Lethaby and Muthesius that also exposes a significant divergence in the tendency of their ideas. Their ideas led directly to the architecture of the early modern period. A final comparison of early modern and pre-Victorian ideas and buildings exposes the limitations of modern architectural theory.