An Extensional Modified Realizability Topos Mees de Vries Abstract: In this thesis, we construct and investigate a topos for Kreisel’s modified realizability. The topos, like Kreisel’s modified realizability, is characterized by the axiom of choice in all finite types and the principle of independence of premise. The model is constructed by a general method known as the tripos-to-topos construction. It is closely related to an existing topos, constructed for a modification of modified realizability by Troelstra, usually also called modified realizability. We pay special attention to the subcategory of our topos on ¬¬-separated objects, constructing it separately. This category is more accessible, and the logical features we look for in our topos are already present in this smaller category.