  In this thesis we investigate some aspects of the dramatic consequences of
supersymmetry breaking on string vacua. In particular, we focus on the issue of
vacuum stability in ten-dimensional string models with broken, or without,
supersymmetry, whose perturbative spectra are free of tachyons. After
formulating the models at stake in Chapter 2, we introduce their low-energy
effective description in Chapter 3, presenting a number of vacuum solutions to
the classical equations of motion. In Chapter 4 we analyze their classical
stability, studying linearized field fluctuations, and in Chapter 5 we turn to
the issue of quantum stability. In Chapter 6 we frame the resulting
instabilities in terms of brane dynamics, examining brane interactions and
back-reacted geometries. In Chapter 7 we propose a holographic correspondence
connecting bulk instabilities with dual renormalization group flows, and we
explore a potentially concrete scenario involving world-volume gauge theories.
Finally, in Chapter 8 we turn to cosmology, deriving generalized no-go results
for warped flux compactifications and concocting a brane-world scenario along
the lines of a recent proposal, providing a string-theoretic embedding of
constructions of this type. In Chapter 9 we provide a summary and collect some
concluding remarks.
