Progressive multifocal leukoencephalopathy (PML) is a disease of immunosuppressed patients caused by the JC polyomavirus (JCPyV). Due to the elevated risk in patients treated with natalizumab for multiple sclerosis (MS) and also treatment with other biologicals for different indications, the relevance of PML has increased in recent years. This article summarizes the published knowledge on the biology and pathogenesis of PML with a focus on the role of cerebrospinal fluid diagnostics in the work-up for PML and the current PML case definition. Current recommendations regarding risk management are discussed, as are possible therapies and prevention.