Gerundials and infinitvals are internally structured like S's in the sense that they formally show aspectual distinctions, preserve functional notions, can incorporate adverbials and negative particles, have active and passive verbal forms, and can show temporal distinctions. There are, however, numerous restrictions on the type of S that may underlie a gerundial or an infinitival. To account for these restrictions satisfact-orily, we must set up certain types of constraints, in partic-ular: deep structure constraints, surface structure con-straints, and transformational constraints. Though internally structured like ans, gerundials and infinitivals are shown to have the same distribution as an NP, and in most of their occurrences to behave typically like an NP under transformations. In some contexts, however, infinitivals and certain types of -ing constructions that are often treated as gerundials do not behave like an NP. This incon-sistency in the syntactic behaviour of the two types of clause should not be taken as evidence that certain S's are generated under the domination of the category NP whereas others are not. Greater simplicity would accompany an analysis that generates all S's underlying gerundials and infinitivals under the dom-ination of an NP. The inconsistency in. the syntactic be-haviour of gerundials and infinitivals is accounted for in terms of S- Pruning and other constraints. For the theory to handle the syntactico-semantic con-trasts that headed and headless sentential complements exhibit, complementizers should be generated in deep structure. More-over, headless gerundi a l s and infinitivals should be generated as the sole constituent of the dominating category Complement, which is immediately dominated by the node NP. Nonetheless, in order to predict the occurrence of certain headless gerund-i als and infinitivals and the non-occurrence of others, the node Comp should share the constituency of the dominating cate-gory NP with an N that consists solely of semantic features (i.e. it bas no phonetic realization). The preform it that appears in the surface structure of certain S's incorporating a sentential complement i s not generated in deep structure but is treated as a pronominal remnant of a copied complement. The various morphological shapes that gerundials and infinitivals assume in surface structure result from the application of certain transformations (most of which are in-dependently motivated) to underlying sentences incorporating the gerundial or the infinitival complementizer. In this respect it is argued that the gerundial complementizer is the suffix -ING and that the infinitival complementizer is the morpheme To.