Mandarin Chinese is said to be a radical pro-drop language, in the sense that verbal arguments in this language can be dropped rather freely. However, in this dissertation, I show that the omission of arguments in Mandarin Chinese is in fact constrained by various conditions. First, I demonstrate that the availability of a discourse topic is insufficient to license empty categories in Mandarin Chinese by showing that subject and object positions cannot be left empty at random. Some empty subject positions are neither true instances of nominal ellipsis nor variables bound by discourse topics; instead, they are a side effect of verb or vP movement followed by TP-ellipsis. Next, I address the issue of when objects can be "dropped" in Mandarin Chinese. I argue that structural parallelism built on verbal identity between sentences plays an important role in licensing `objectless' sentences. I propose that the mechanism responsible for the creation of such sentences is V-stranding VP-ellipsis rather than argument ellipsis. In the last part of this dissertation, I show that, although we cannot rely on the strength of discourse alone to account for empty categories, the concept of topic-hood is nevertheless implicated in the appearance of certain empty argument positions in sentences used in monologues. I claim that subject pro in Mandarin Chinese must have as its antecedent an element located in an A'-position, which can be overt or covert. In addition, I suggest that the differences between Italian, Japanese, and Mandarin Chinese with respect to the use of subject pro can be boiled down to the featural properties of the covert topic TOP preceding subject pro: this covert topic has inherently valued phi-features in Italian and Japanese, while its counterpart in Mandarin Chinese does not. / Linguistics