Psychotic disorders are among the most expensive brain-related disorders in Europe. This is mainly due to their onset early in life and their long-term disabling courses. Current treatments fail to improve most influential factors such as social-cognitive deficits. Prevention is recognized as one of the key strategies to fight these deteriorating outcomes and is expected to significantly reduce both, the societal costs as well as the immense burden for the patients and for their families. Recent meta-analyses indicate promising preventive effects of both pharmacological and cognitive-behavioural interventions. Yet, reported transition rates are still too high. Clinical evidence suggests that disturbances of social functioning predict conversion to psychosis. Neurobiological evidence implicates glutamatergic dysfunction and redox imbalance in the pathophysiology of schizophrenia. We hypothesize that interventions targeting (i) social functioning and (ii) glutamatergic / oxidative pathways already in at-risk states would significantly reduce transition rates. To test these hypotheses, our study is designed as a randomized, placebo-controlled, 18-month trial (six months of intervention plus 12 months of follow-up), involving 200 subjects at-risk for psychosis. Specifically, we will compare the preventive effects of a cognitive-behavioural and social-cognitive intervention to a pharmacological intervention (IPPI) with Acetylcysteine, a drug with a proglutamatergic, neuroprotective and anti-inflammatory profile in a 2x2 factorial design. The results of our planned study are expected to provide new and well tolerated interventions, thus hopefully helping to achieve the major goal of individualized prevention, and, consequently, lower the individual and societal burden of psychosis.