Training in mindfulness, the non-judgmental observation of experiences as they arise in the present moment, has been increasingly and successfully applied to the treatment of normative stress conditions and mental disorders. Yet, the neurological mechanisms that underlie the reported improvements are still largely unknown. This longitudinal study will investigate the influence of mindfulness training on a key underpinning of mental health, namely emotion regulation, and its associated brain activity. Healthy participants will be randomly assigned to either a validated eight week Mindfulness-based stress reduction (MBSR) program or to a control condition. In a pre-post investigation, participants’ subjective reactions to aversive emotional stimuli (affective facial expressions) will be assessed, as will the associated brain activation using fMRI. We hypothesize that after the training the MBSR participants will rate the pictures as less aversive compared to control participants. Furthermore, the MBSR participants will show a patter of brain activation indicative of improved emotion regulation, relative to control participants. Finally, the effect of MBSR on the gray matter structure of the brain will be investigated.