  One of the main goals of the field of string phenomenology is to describe the
properties of our spacetime in low-energy effective models related to and
consistent with string theory. It is known that our universe undergoes
accelerated expansion, which can be described in Einstein's theory of general
relativity as a positive cosmological constant. Within string theory
descriptions, this is realized by a scalar field with a positive value of its
potential. Oftentimes, this is modeled by having the scalar at a stable,
positive minimum. One then obtains a de Sitter spacetime. Constructions of de
Sitter spaces from string theory are not straightforward and face many
criticisms. In this thesis, we address several aspects related to this topic.
We investigate the description of general branes in supergravity and in
particular describe the uplifting anti-D3-brane of the KKLT model, including
all world-volume fields. Additionally, we translate the type IIB based KKLT
model into type IIA and introduce a mechanism that allows for the rapid
construction of many de Sitter solutions. Related to this, we investigate
models based on twisted 7-tori from M-theory and their relation to type II
supergravity. These setups can also yield de Sitter solutions using the same
mass production mechanism. One of the more recent and serious claims against
the construction of de Sitter spaces in string theory, the de Sitter swampland
conjecture, is also addressed here. We highlight the difficulty of obtaining a
stable, positive vacuum energy in classical type IIA flux compactifications and
their reliance on O-plane sources. Then, we turn our attention to the
conjecture itself. Contrary to the initial claim, we find many unstable de
Sitter points in numerical searches and go on to present our own refined
conjecture that differs in several key aspects from the refined version of the
original conjecture.
