We briefly outline the rationale for employing positron emission tomography (PET), using the ligand [¹¹C](R)-PK11195, the binding site for which is highly expressed by activated microglia, in order (a) to detect in vivo neuroinflammatory changes occurring in a variety of brain diseases and at different disease stages and (b) to monitor the progression of neuroinflammation as a generic in vivo marker of ‘disease activity’. The use of [¹¹C](R)-PK11195 PET is described as a systematic attempt at measuring the emerging phenomenology of tissue pathology itself—as opposed to measuring, for example, the loss of neuronal function or structure—and as a proof of principle for the clinical utility of imaging glial cells in vivo.