This thesis investigates the Spanish subject position at the interfaces between syntax, semantics, morphology, and pragmatics from a derivational perspective. One major aim is to show that (null) subjects in finite pro-drop and nonfinite control are not different with respect to their internal structure and that the mechanisms responsible for yielding the interpretative (LF) as well as morpho-phonological (PF) shape of the subject position are governed by similar principles in the two configurations. This way, PRO and pro do not exist as independent theoretic constructs, but they constitute derivational occurrences of the same underlying nominal element. An analysis of Spanish null subjects as well as overt, strong pronouns is proposed that considers them the result of merger of a lexically underspecified D[ϕ:_] element which ‘grows’ during the syntactic derivation by means of entering dependencies with the functional heads v and C and being assigned pragmatic features at the phase edges. This way, the nature of the Spanish subject position is not determined by lexical specification of the subject, but it is derived by means of an interaction between ϕ-Agree operations, C-related discourse linking, and assignment of [topic] and [focus] features at the vP-phase level. A main assumption will thus be that features like [topic] and [focus] can take over functions in the interpretative as well as morpho-phonological construction of Spanish subjects that are fulfilled by morpho-syntactic features like ϕ and Case in English. Among the constructions that will be explained by means of this reasoning are (i) obligatorily (English) vs. optionally (Spanish) empty subjects in complement control infinitives, (ii) obligatorily overt (English) vs. optionally null (Spanish) subjects in finite clauses, (iii) the alternation between strong pronouns with bound and disjoint interpretations in Spanish embedded clauses (iv) emphatic ‘doubling’ configurations in Romance NSLs, (v) apparent Backward Control phenomena, and (vi) some configurations in the context of the personal infinitive in Spanish.