This thesis presents researches and experiments performed in collaboration with a psychiatrist in order to validate and improve the use of virtual reality in social phobia psychotherapy. Cognitive and behavioral therapies are strongly based on the exposure to anxiety provoking stimuli. Virtual reality seems to be appropriate for such exposures as it allows for on-demand reproduction of reality. The idea has been validated for the treatment of various phobias but is more delicate in the case of social phobia; whereas the sense of presence provoked by the immersion in a virtual environment supports the emergence of fears linked to a location, we had to verify that we can reproduce social phobia related anxiety-provoking stimuli by simulating virtual humans. Therefore, and in order to provide therapists with an efficient virtual reality system dedicated to the exposure to social situations, we have developed software solutions supporting different immersion setups and enabling realistic simulations of inhabited virtual environments. We have experimented with public speaking scenarios within a preliminary study, three clinical case studies and a validation study on 200 subjects. We have been able to confirm that our virtual reality platform fulfilled therapeutic exposure requirements for social phobia. Moreover, we have been able to show that virtual reality exposure has additional advantages such as the possibility to improve clinical assessment with embedded monitoring tools. Our experiments with physiological measurements and eye tracking technology during immersion leaded to the validation of systems for objective and reliable assessment of patients' safety behaviors. The observation of such phobic reactions has confirmed the simulation impact and may provide therapists with enhanced pathological progression monitoring. During our experiments, we have also been able to observe that subjects' reactions during immersion were so much influenced by their sensitivity to fearful stimuli that their cognitive reactions were 'overloaded' by the arousal of anxiety and emotions. This has allowed us to consider that the sense of presence was more importantly related to the subjective impact of the content than to the technological process.