Background and purpose: New‐onset refractory status epilepticus (NORSE) is a clinical presentation, neither a specific diagnosis nor a clinical entity. It refers to a patient without active epilepsy or other pre‐existing relevant neurological disorder, with a NORSE without a clear acute or active structural, toxic or metabolic cause. This study reviews the currently available evidence about the aetiology of patients presenting with NORSE and NORSE‐related conditions.Methods: A systematic search was carried out for clinical trials, observational studies, case series and case reports including patients who presented with NORSE, febrile‐infection‐related epilepsy syndrome or the infantile hemiconvulsion‐hemiplegia and epilepsy syndrome.Results: Four hundred and fifty records were initially identified, of which 197 were included in the review. The selected studies were retrospective case–control (n = 11), case series (n = 83) and case reports (n = 103) and overall described 1334 patients both of paediatric and adult age. Aetiology remains unexplained in about half of the cases, representing the so‐called ‘cryptogenic NORSE’. Amongst adult patients without cryptogenic NORSE, the most often identified cause is autoimmune encephalitis, either non‐paraneoplastic or paraneoplastic. Infections are the prevalent aetiology of paediatric non‐cryptogenic NORSE. Genetic and congenital disorders can have a causative role in NORSE, and toxic, vascular and degenerative conditions have also been described.Conclusions: Far from being a unitary condition, NORSE is a heterogeneous and clinically challenging presentation. The development and dissemination of protocols and guidelines to standardize diagnostic work‐up and guide therapeutic approaches should be implemented. Global cooperation and multicentre research represent priorities to improve the understanding of NORSE.