Mono-aminergic neurotransmitters, in particular serotonin (5-HT), are involved in regulating a large number of psychological and physiological functions, and abnormal 5-HT transmission has been implicated in a wide variety of neuropsychiatric disorders. Positron emission tomography (PET) is a non-invasive nuclear imaging technique with exquisite sensitivity and specificity, allowing delineation of neurotransmitter function in vivo. Over the last two decades, PET has been used to investigate 5-HT function in several neuropsychiatric disorders, in particular mood and anxiety disorders, schizophrenia, Alzheimer's disease, and impulse control disorders. In the present review, an overview of recent findings is provided, and possibilities for further research are discussed.