Brain ischemia has been recently a central problem in basic and applied studies in angioneurology. The latest investigations that give an insight into a relationship between metabolic changes and cerebral blood flow and make it possible to study ischemia at the molecular level and its changes over time have promoted the accumulation of fundamentally new facts and some reappraisal of existing ideas. Ischemic semishadow or penumbra is one of the most important presently studied phenomena. Detection of penumbra signs suggests that it is expedient to evaluate cerebral blood flow and metabolism when planning treatment (thrombolysis or neuroprotective therapy) and that it is important to predict the severity of ischemic stroke. Positron emission tomography (PET) is the reference method for detecting the penumbra; however, its application is limited in clinical practice. Computed tomography (CT) perfusion and perfusion MRI, a combination of diffusion-weighted and perfusion MRI, single-photon emission CT, and xenon-enhanced CT are most frequently used to evaluate a cerebral ischemia area and its blood flow. However, there are no standardized approaches to quantifying the thresholds for cerebral blood flow or unified algorithms for penumbra verification, which calls for further investigations.