Status epilepticus (SE) describes an enduring epileptic state during which seizures are unremitting and tend to be self-perpetuating. We describe the clinical phases of generalized convulsive SE, impending SE, established SE, and subtle SE. We discuss the physiological and biochemical cascades which characterize self-sustaining SE (SSSE) in animal models. At the transition from single seizures to SSSE, GABA(A) (gamma-aminobutyric acid) receptors move from the synaptic membrane to the cytoplasm, where they are functionally inactive. This reduces the number of GABA(A) receptors available for binding GABA or GABAergic drugs, and may in part explain the development of time-dependent pharmacoresistance to benzodiazepines and the tendency of seizures to become self-sustaining. At the same time, 'spare' subunits of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and NMDA (N-methyl-D-aspartic acid) receptors move from subsynaptic sites to the synaptic membrane, causing further hyperexcitability and possibly explaining the preserved sensitivity to NMDA blockers late in the course of SE. Maladaptive changes in neuropeptide expression occur on a slower time course, with depletion of the inhibitory peptides dynorphin, galanin, somatostatin and neuropeptide Y, and with an increased expression of the proconvulsant tachykinins, substance P and neurokinin B. Finally, SE-induced neuronal injury and epileptogenesis are briefly discussed.