This lecture introduces two major architectural movements of the 20th century — Art Deco and Modernism, including the emergence of the International Style. It explains how Art Deco developed in the interwar period (1925–1940) as a bold, geometric, decorative style that embraced technology, luxury, and industrial progress. Through examples like the Chrysler Building and Empire State Building, the lecture highlights key features such as stepped facades, smooth wall surfaces, geometric ornamentation, and streamlined forms. The spread of Art Deco to Mumbai and its Indo-Deco adaptations are also discussed. The lecture then transitions to Modernism as a broader movement that rejected ornament and emphasized function, technology, and simplicity. Core ideas such as “form follows function,” truth to materials, and integration of art and technology are explained along with their proponents like Louis Sullivan, Adolf Loos, Le Corbusier, Walter Gropius, and Mies van der Rohe. The session concludes with the rise of the International Style, its technological foundations in steel, RCC, and glass, and its transformation into corporate modernism and global skyscraper architecture.