This dissertation is concerned with the specific relationship between copular sentences and existential sentences, and what about that relationship is responsible for the well-known restrictions on definiteness and predicates that are observed in the existential cases. Instead of starting with the well-known paradigms which illustrate restrictions in the existential constructions that result in ungrammaticality; this dissertation starts by trying to account for the more apparent flexibility the existential construction has when it comes to agreement. While previous accounts of these phenomena have sought characterize such agreement phenomena as marked or some sort of default agreement or just a PF/phonological phenomena with no semantic/pragmatic implications, I will argue the opposite. I will suggest that the singular/plural agreement is a result of the mass/count properties of the Predicational Structure from which the sentence is derived. Furthermore, I will suggest that it is precisely these mass/count properties that account for the observed restrictions in existentials. Additionally, since the theory being advanced here is one that is dependent on mass/count properties, which are universally available in natural languages (albeit encoded and manifested in different ways), it allows us to understand related phenomena in typologically distinct languages, such as English/Mandarin. That is, we can explain why it is that in a "Classifier Language" which encodes mass/count differently than languages like English and French with plural morphology and articles, exhibits the definiteness effect but not the predicate restriction. I will propose that if we understand the existential constructions to be derivationally related to their copular counterparts, and we have a sufficiently fine-grained understanding of predication in copular constructions, then we can not only account for the behavior and restrictions of existentials vis-a-vis copulars within a language, but we can understand the difference between restrictions on existentials across typologically distinct languages in a unified and principled way. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]