The classic concept of the viability thresholds of ischemia differentiates between two critical flow rates, the threshold of electrical failure and the threshold of membrane failure. These thresholds mark the upper and lower flow limits of the ischemic penumbra which is thought to suffer only functional but not structural injury. Recent studies of the functional and metabolic disturbances suggest a more complex pattern of thresholds. At declining flow rates, protein synthesis is inhibited at first (at a threshold of about 0.55 ml/gm/min), followed by a stimulation of anaerobic glycolysis (at 0.35 ml/gm/min), the release of neurotransmitters and the beginning disturbance of energy metabolism (at about 0.20 ml/min), and finally the anoxic depolarization (< 0.15 ml/gm/min). The penumbra, as defined by the classic flow thresholds, does not remain viable for extended periods. Since viability of the tissue requires maintenance of energy-dependent metabolic processes, penumbra is redefined as a region of constrained blood supply in which the energy metabolism is preserved. Imaging of the penumbra by combining autoradiographic cerebral blood flow measurements with bioluminescent images of adenosine triphosphate (ATP) demonstrates a gradual expansion of the infarct core (in which ATP is depleted) into the penumbra until, after a few hours, the penumbra has disappeared. It is suggested that the limited survival of the penumbra is due to periinfarct depolarizations, which result in repeated episodes of tissue hypoxia, because the increased metabolic workload is not coupled to an adequate increase of collateral blood supply. This explains pharmacological suppression of periinfarct depolarizations lowering the threshold of metabolic disturbances and reducing the volume of the ischemic infarct.