There are many arguments that String Theory - the most compelling candidate for a unified "Theory of Everything" - predicts the existence of a huge number of possible four-dimensional worlds, or possible universes. In such a worldview, our universe is but one of a multitude of universes, and the physical laws that govern it are seen as environmental accidents.

The purpose of this project is to demonstrate that one of the key ingredients that enter in the construction of multiple deSitter universes in string theory - the uplift of the vacuum energy - is incorrect, and thus string theory does not have a landscape of deSitter vacua. I intend to to this by first analyzing the most popular uplift mechanism, proposed by Kachru, Kallosh, Linde and Trivedi, and trying to demonstrate by precise string-theory calculations that one of the key assumptions of this mechanism: that anti-D3 branes in a Klebanov-Strassler solution give rise to a metastable vacuum - is not correct. I will then extend this to other uplift mechanisms. This would establish that string theory does not have a landscape of deSitter vacua, which would restore the predictability of string theory, and would weaken the case for an anthropic understanding of our Universe.