The peripheral benzodiazepine receptor (PBR) is expressed on the outer mitochondrial membrane of activated microglia and is implicated in the pathophysiology of a variety of central nervous system and peripheral diseases. The abundant receptor concentration makes PBR a potential biomarker and an attractive target for quantification in vivo using positron emission tomography. PBR can be an important target for monitoring disease progression, for evaluating the effect of therapy, and for investigating new treatment modalities. PBR is also emerging as a potential target in the treatment of neuroinflammatory and neuropsychiatric disorders. Here, we review the positron emission tomography radioligands employed for imaging PBR in living brain and their applications.