In the last decade, positron emission tomography (PET) and single-photon emission computed tomography (SPECT) in vivo imaging has attempted to demonstrate the presence of neuroinflammatory reactions by detecting the 18 kDa translocator protein (TSPO) in many diseases of the central nervous system. Of these diseases, Alzheimer's disease has been the most widely studied with more than forty pre-clinical and clinical studies. Thus, it is now more or less accepted that TSPO is upregulated in Alzheimer's disease. However, studies differ as to the cerebral location of the variations, the timecourse of the appearance of TSPO and the cells responsible for this increase. In the case of schizophrenia, while studies remain more contradictory, it seems that TSPO shows decreases in expression instead. This review focuses on the use of TSPO as a biomarker of neuroinflammation in psychiatric disorders, on the cellular origin of the variations in its expression, on its possible biological/pathological roles and, finally, on its links with other neurochemical actors of Alzheimer's pathology, b-amyloid and neurofibrillary tangles.