When used in combination with specific radioactive markers, functional imaging modalities such as SinglePhoton Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) enable thevisualization of several neurotransmitter receptors and transporters, as well as of the perfusion andmetabolism of the brain.This paper gives an overview of the functional imaging techniques, as well as of the studies that have beenperformed on humans and canines with anxiety disorders. Thus far, most of the research in this field has beenfocused on brain perfusion and the serotonergic and dopaminergic neurotransmitters, and less on gammaaminobutyricacid (GABA), glutamate, norepinephrine and the hypothalamic-pituitary-adrenal (HPA) axis.