Posttraumatic stress disorder (PTSD) is a common and very debilitating disorder. Current treatment strategies for PTSD yield suboptimal effects: irrespective of treatment strategy, fifty percent of PTSD patients remain symptomatic after treatment. Knowledge of the processes that account for therapeutic change is imperative to improve treatment efficacy. One of the first-line treatments for PTSD is exposure therapy, the clinical counterpart of extinction. The crucial insight is that extinction of fear is not based on the weakening of threat associations, but on the learning of non-threat associations that compete with the threat associations. This is referred to as inhibitory learning. For exposure therapy to be long-term successful, non-threat associations need to be learned and remembered across time and contexts. Studies in animals, healthy-human samples and simple phobias have provided new ideas on how inhibitory learning can be enhanced. These ideas have not yet been tested in PTSD. The central question of this project is: Can exposure therapy for PTSD be optimized by maximizing inhibitory learning? The enhancement strategies that yield most promise to augment exposure effects are 1) enhancing expectancy violation, and 2) maximizing variability. Building on my expertise on exposure therapy for PTSD, I propose to investigate these enhancement strategies in a clinical PTSD sample. In three studies, I will test the effects of expectancy violation, stimulus and fear variability, and context variability on exposure outcome. By using an innovative design, the effects of these augmentation strategies on a brief exposure intervention (clinical assay) will be assessed using psychophysiological, behavioural, and verbal outcome measures. This project will directly test the effect of manipulating inhibitory learning on exposure success. Findings will impact the delivery of exposure therapy for PTSD and contribute to improved treatment efficacy for those suffering from PTSD.