Selective serotonin reuptake inhibitors (SSRIs) raise serotonin (5-HT) in the synaptic cleft and are the current first line of pharmacological antidepressive treatment. Yet, there is a missing link between this first molecular step in their mechanism of action and observed clinical improvement. We have determined to establish a framework combining genuine molecular and functional imaging, i.e. hybrid pharmaco-PET/MR imaging, of the human serotonergic system in order to predict antidepressant treatment response.