Résumé : The transitioning of the boundary layer around atmospheric entry vehicles from a laminar to a turbulent state leads to an increase of the surface heat flux of up to an order of magnitude. The uncertainty on where this transitioning occurs ultimately leads to non-optimal vehicle designs, and the resulting loss of available payload, or to catastrophic mission failure. This dissertation extends the state of the art of boundary-layer stability and transition in the presence of ablation phenomena such as surface outgassing, air chemistry, and gas-surface interaction. Simplified models are developed for both the laminar-base-flow and the instability problems. For the latter, a combination of linear stability theory and the semiempirical eN method are employed. These models are then deployed on a variety of test cases in order to address several research voids. The eN method is seen to correctly predict experimentally-obtained transition-onset locations in wall-blown scenarios. The use of a porous wall boundary condition provides better perturbation-growth estimations than a homogeneous condition. The injection through the wall of gases lighter than air is seen to advance transition due to the introduction of a shocklet that acts as a thermoacoustic impedance, coupled with the porous surface's admittance. Second-mode waves are seen to be strongly affected by the arrangement of the porous surface, the effect of which could vary from strongly destabilizing to stabilizing depending on the pore size and the porous-layer height. An extensive study is performed on the sensitivity of instability-characteristic predictions to high-enthalpy modeling. The predictions are seen to be most sensitive to the use of models that result in an erroneous boundary-layer height, being the transport model the one with the largest influence. The excitation of internal energy modes and the dissociation of molecular species is seen to have a destabilizing effect on second-mode waves, whilst ionization has the opposite effect. Other ablation-induced phenomena like the injection of carbon species, are also generally destabilizing. Three competing effects are observed in reacting boundary layers: the chemistry-induced cooling of the base flow destabilizes, whilst diffusion fluxes and chemical source terms acting on the perturbations stabilize. Unstable supersonic modes in strongly-reacting hot-wall scenarios are seen to be promoted by perturbation diffusion fluxes, rather than base-flow cooling, as was the common understanding.