In health, microglia reside as quiescent guardian cells ubiquitously, but isolated without any cell‐cell contacts amongst themselves, throughout the normal CNS. In disease, however, they act as swift “sensors” for pathological events, including subtle ones without any obvious structural damage. Once activated, microglia show a territorially highly restricted involvement in the disease process. This property, peculiar to microglia, confers to them diagnostic value for the accurate spatial localisation of any active disease process, acute or chronic. In the brain, the isoquinoline PK11195, a ligand for the peripheral benzodiazepine binding site (PBBS), binds with relative cellular selectivity to activated, but not resting, microglia. Labelled with carbon‐11, (R)‐PK11195 and positron emission tomography (PET) have been used for the study of inflammatory and neurodegenerative brain disease in vivo. These studies demonstrate meaningfully distributed patterns of regional [¹¹C](R)‐PK11195 signal increases that correlate with clinically observed loss of function. Increased [¹¹C](R)‐PK11195 binding closely mirrors the histologically well‐described activation of microglia in the penumbra of focal lesions, as well as in the distant, anterograde, and retrograde projection areas of the lesioned neural pathway. There is also some indication that in long‐standing alterations of a neural network with persistent abnormal input, additional signals of glial activation may also emerge in transsynaptic areas. These data suggest that the injured brain is less static than commonly thought and shows subtle glial responses even in macroanatomically stable appearing regions. This implies that glial activation is not solely a sign of tissue destruction, but possibly of disease‐induced adaptation or plasticity as well. Whilst further technological and methodological advances are necessary to achieve routine clinical value and feasibility, a systematic attempt to image glial cells in vivo is likely to furnish valuable information on the cellular pathology of CNS diseases and their progression within the distributed neural architecture of the brain. GLIA 40:206–217, 2002. © 2002 Wiley‐Liss, Inc.