In this chapter, two autoimmune disorders against astrocytes are reviewed: neuromyelitis optica (NMO), recently renamed NMO spectrum disorders (NMOSD) which frequently associates with antibodies against AQP4; and a meningoencephalitis without specific clinical or brain MRI features, which occurs in association with antibodies against glial fibrillary acidic protein (GFAP) that are mainly detected in CSF. NMOSD should be considered a syndrome rather than a disease because it associates with different antibodies and pathogenic mechanisms. In epidemiological studies, 73% of patients with NMOSD have aquaporin 4 (AQP4) antibodies whereas 12% harbour myelin oligodendrocyte glycoprotein (MOG) antibodies and 15% are seronegative. The diagnosis of NMOSD is based on clinical and laboratory criteria that require the presence of one or two core clinical syndromes, depending on the positivity of AQP4 antibodies. Relapses in NMOSD are common and several treatments with monoclonal antibodies have shown efficacy, including the following targets: the C5 complement protein (eculizumab), the interleukin 6 receptor (satralizumab), and the CD19 B cell receptor (inebilizumab). The pathogenic role of AQP4 antibodies is supported by the neuropathological findings in patients with NMOSD, and by in-vitro studies and passive transfer of antibodies to experimental animals.