This dissertation investigates the distribution and interpretation of covert modality. Three environments where covert modality appears are analyzed. These environments are infinitival relative clauses, infinitival questions, and ability modals. Infinitival relative clauses are shown to not form a unified class structurally. Subject infinitival relative clauses are assimilated to the class of reduced relative clauses. They lack a CP projection. Non-subject infinitival relative are assimilated to the class of full relative clauses. Like full relatives, they have a CP projection. The infinitival [+wh] Cº is argued to be the source of the modality in a non-subject infinitival relative clause. All nonsubject infinitival relative clauses and infinitival questions involve modality because of the obligatory presence of the infinitival [+wh] Cº. Since subject infinitival relative clauses do not involve the infinitival [+wh] Cº, they are not necessarily modal. If they are modal, the source of the modality lies within the infinitival clause. The conditions under which a subject infinitival relative can receive a non-modal interpretation are analyzed. It is shown that a non-modal interpretation must be licensed and that only a limited class of modifiers (superlatives, ordinals, and only) in a particular configuration can license the non-modal interpretation. The licensing configuration obtains under reconstruction of the head NP of the relative clause. The basic result regarding themodality in infinitival questions is that despite the apparent variability in the nature (deontic vs. circumstantial) and the force of this modality, we really have just one modality. The apparent variation in force and nature falls out from the interaction between the semantics of the infinitival [+wh] C0 and contextual factors. The apparent variation in the force and nature of the infinitival questionmodality is also found in non-subject infinitival relative clauses. The proposal for capturing variable modality effects in infinitival questions is extended to non-subject infinitival relatives. A covert modal (the Generic operator) is responsible for the modality in an ability modal. The ability modal itself has the semantics of an implicative verb like manage.