Abstract : This thesis explains how the adjunction of three features to System Fω allows writing programs in a modular way in an explicit system à la Church, while keeping a style that is similar to ML modules. The first chapter focuses on open existential types, which provide a way to consider existential types without scope restrictions: they permit to organize programs in a more flexible manner. The second chapter is devoted to the study of singleton kinds, which model type definitions: in this framework, we give a simple characterization of type equivalence, that is based on a confluent and strongly normalizing reduction relation. The last chapter integrates the two previous notions into a core language equipped with a subtyping relation: it greatly improves the modularity of Fω to a level that is comparable with the flexibility of ML modules. A translation from modules to this core language is defined, and is used to precisely compare the two languages.