Majorizability Types, Assemblies, and the Fan Theorem Tao Gu Abstract: In this thesis we introduce the notion of majorizability types, and the category MMAsm of majorizability modified assemblies. The MMAsm has the vdBB model as the interpretation of finite types. The internal logic of MMAsm captures certain version of modified realizability. Besides, both the Special Fan Functional and the Fan Recursor provably exist in the internal logic of MMAsm. Therefore the Fan Theorem holds in MMAsm. And this provides a computational and categorical justification of the Fan Theorem.