This paper analyzes the modularization in the world auto industry. The modularization in the industry has involved architectural changes in product, production, and supplier systems with each region (Japan, Europe and the U.S.A.) emphasizing different purposes and aspects. As an attempt to understand such multi-faceted, complex processes coherently, this paper proposes a conceptual framework that sees development / production activities as interlinked, multiple hierarchies of products, processes, and inter-firm boundaries. With this framework, drawing on case studies and questionnaire survey data, the paper examines the on-going processes of modularization in the industry. It is argued that tensions exist among the three hierarchies, and such tensions may lead to further changes in product, production and supplier-system architectures in the auto industry, in a dynamic and path-dependent manner.