Grammatical theories that rely exclusively on the categorical nature of constituents to determine their syntactic behavior encounter problems when dealing with cleft construction. The ungrammaticality of such constructions is indeed syntactic in nature and can be shown to derive from a general principle of universal grammar (UG), restricting the range of null operators that partake in cleft construction. Theoretical assumptions underlying the clefting phenomena are reviewed, followed by a demonstration of the plausibility of a descriptive generalization concerning cleftability stated in terms of Theta Theory (Chomsky, 1981). Several apparent counterexamples to that generalization are considered which are shown to fall out of other modules of the grammar developed within the Government Binding framework. The question of why such a generalization should hold is addressed through deeper insight into the characteristics of the null operator involved in the clefting construction. A constraint is formulated regarding the range of null operators. It is proposed that this Null Operator Generalization be extended to all base-generated empty categories, a hypothesis which is shown to rightly predict the distribution of predicate clitics in French. (CB)