Growing evidence derived from cerebrospinal fluid (CSF), neuropathological, imaging, genetic, and epidemiological studies link neuroinflammation and immune dysregulation to a subset of individuals with a variety of severe mental disorders (SMDs), including affective and non-affective psychotic disorders. Further, the recent discoveries of neuronal surface antibodies (NSAs) in autoimmune encephalitis (AE) presenting with diverse neuropsychiatric disorders such as psychosis and cognitive decline, among many others, provides further support to the notion that CNS autoimmunity and neuroinflammation can contribute to the neurobiology of psychiatric disturbances. Further, these immune mechanisms may contribute to a subset of patients currently diagnosed as having treatment-resistant SMDs such as schizophrenia and major depressive disorder. Additionally, mounting data indicate that various infections can serve as an immunological trigger of aberrant immune responses, presumably by causing release of excess neural antigen, thereby giving rise to NSAs or aberrant immune cellular responses to give rise to primary or secondary psychiatric disorders such as schizophrenia and those associated with AE, respectively. Collectively, these findings support the “mild encephalitis” hypothesis of SMD. The significant overlap among AE-associated psychosis, systemic autoimmune disorder-associated psychosis, and psychotic disorders associated with pathological processes involving inflammation and immune dysregulation has also prompted some authors to adopt the term “autoimmune psychosis” (AP). This term reflects that this psychosis subtype is mechanistically linked to complex neuroimmune and inflammatory signalling abnormalities that can be responsive to early immunomodulatory treatment. It also suggests that a subset of AP might represent an incomplete or “forme fruste” subtype of AE presenting with dominant or pure psychiatric symptoms mimicking primary psychiatric illnesses. Because data indicate that delayed diagnosis and treatment may lead to permanent sequelae, early recognition of AP utilizing neurodiagnostic workup (e.g., CSF analysis, neuroimaging, and EEG) and its early treatment with appropriate immunotherapy are paramount to a meaningful recovery. This eBook will provide an overview of the current knowledge and research areas from epidemiology, risk factors and diagnosis to the management of these conditions, in this rapidly emerging field, helping to bridge the gaps in knowledge that currently exist in the disciplines of Psychiatry, Neurology, and Neuroimmunology.