  The subject of this thesis are various ways to construct four-dimensional
quantum field theories from string theory. In a first part we study the
generation of a supersymmetric Yang-Mills theory, coupled to an adjoint chiral
superfield, from type IIB string theory on non-compact Calabi-Yau manifolds,
with D-branes wrapping certain subcycles. The low energy limit of this
non-Abelian gauge theory can be obtained from a second non-compact Calabi-Yau
geometry, which is related to the first one through a geometric transition. In
particular, the effective superpotential governing the vacuum structure of the
gauge theory can be obtained from integrals on a Calabi-Yau manifold. These
integrals in turn are related to matrix model quantities and one therefore can
use the matrix model to learn something about the gauge theory vacua. The
second part of this work covers the generation of four-dimensional
supersymmetric gauge theories, carrying several important characteristic
features of the standard model, from compactifications of eleven-dimensional
supergravity on G_2-manifolds. We discuss anomaly cancellation through inflow
in the case of conical singularities, present an explicit compact manifold with
two conical singularities and weak G_2-holonomy, and review the anomaly
cancellation mechanism in the context of M-theory on the interval.
