This thesis explores the nature of the empty category PRO within the Minimalist Program of Chomsky (1995). It is proposed that the distribution of PRO follows from Case theory, hence effectively eliminating the need for the notion government for this purpose. I argue, following Chomsky and Lasnik (1993), that PRO is marked with null Case and that null Case is checked only in the domain of certain instances of non-finite T(ense). Crucial to the proposal are arguments that non-finite T in control infinitivals is (+Tense), whereas non-finite T in raising infinitivals is ($-$Tense), as initially suggested by Stowell (1981). From this, the (near) complementary distribution of PRO and lexical DP/trace in the subject position of infinitives can be...