Acute transverse myelitis (ATM), a subgroup of various conditions that cause transverse spinal cord syndromes, is an inflammatory and usually idiopathic spinal cord disease. Idiopathic ATM is diagnosed according to established criteria which mainly assure a clinical picture of bilateral symptoms and signs attributable to spinal cord disease, existence of focal cord inflammation by magnetic resonance imaging and cerebrospinal fluid studies, and exclusion of many diseases, especially cord compression and "disease-associated ATM", as Sjögren disease. Patients with idiopathic ATM have motor, sensory and autonomic dysfunctions with acute or subacute onset, progressing to nadir in 4 h to 21 days, with full or partial recovery in most cases. Emergent treatment to halt inflammation with corticosteroids is required as well as early appropriate symptomatic therapies. Recent discovery of novel biomarkers has set apart specific inflammatory and autoimmune myelopathies, namely paraneoplastic myelitis and neuromyelitis optica spectrum diseases, the diagnosis of which is crucial to establish therapeutic strategies.