This thesis is a study of clause architecture within a theory of generative grammar. It discusses four major syntactic hypotheses that have crucial bearing on the design of phrase structure: the Agreement-based Case theory, the internal subject hypothesis (ISH), the uniform three-level X-bar theory, and a hypothesis about string vacuous head movement.In Chapter 2 through Chapter 4, I discuss object positions in three typologically and genetically different languages, i.e. English, Zarma, and Japanese, and argue that they all possess Object Agreement Phrases (AGRoP). A consideration of learnability suggests that the presence of AGRoP in these languages, particularly in Japanese, is not learnable from the data available to children. Thus, it is highly likely that UG is so construed that every language has AGR (THE UNIVERSAL AGR HYPOTHESIS).In Chapter 5, I turn to subject positions. There is conflicting evidence regarding the base-position of the external argument. Some data indicate that the external argument originates in a position lower than its surface position, as is expected under the internal subject hypothesis. Other data suggest that the base-position of the external argument is outside AGRoP and the VP that dominates the main verb and the internal arguments. If the ISH is correct in that all arguments of a predicate category (e.g. V) originate within the maximal projection of this category, then the apparently contradicting data suggest that there are two verbs, hence two VPs, in a single clause (THE SPLIT VP HYPOTHESIS). One verb is above AGRoP, and the other is below AGRoP. The so-called external argument is an argument of the upper V, and "internal arguments" are arguments of the lower V.Chapter 6 is concerned with X-bar theory. I argue, contrary to the wide held view, that some functional categories allow more than one specifier position within their projections (LAYERED SPECIFIERS), and that the number of specifiers is different across categories. If this is correct, the X-bar schema as such cannot be a part of Universal Grammar, as already suggested in Fukui (1986) and others. Our claim, however, is crucially different from Fukui's (1986) in that the specifiers of functional categories do not necessarily (but sometimes do) "close off" their projections.In Chapter 7, I discuss string vacuous verb raising in head final languages. In particular, I present evidence that verbs in Japanese raise out of the VP in overt syntax. Its consequences are also explored to various aspects of syntactic theory such as the Proper