On String Vacua without Supersymmetry

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 introducing 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 brane dynamics, studying 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 potential 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 building a brane-world scenario along the lines of a recent proposal, providing a string embedding of constructions of this type. In Chapter 9 we provide a summary and collect some concluding remarks.