Progressive multifocal leukoencephalopathy (PML) is a rare, opportunistic infection of the CNS by the ubiquitous JC virus (JCV). PML is only seen in the context of severe and prolonged immunosuppression, a phenomenon now frequently encountered since the AIDS pandemic. PML is characterised by progressive lysis of oligodendrocytes with demyelination. A rapid clinical course ensues with focal neurological deficits and a median time to death of 3.5 months without treatment. Prior to highly active antiretroviral therapy (HAART), there was no effective therapy. Since the advent of HAART, the prognosis for PML has much improved; however, a significant number of patients appear unresponsive to antiretrovirals and some worsen because of the development of immune reconstitution disease. A better understanding of the biology of JCV and its interactions with host cells is leading to new anti-JCV-specific agents that await evaluation in randomised, controlled trials. Improved diagnostic tools and the possibility of immunotherapy and gene therapy are further advancing the field.