Despite the progress, which has been made in diagnosis and therapy of encephalitis and bacterial meningitis, these acute inflammatory diseases of the brain still display a certain amount of morbidity and mortality. History, physical examination, analysis of serum and cerebrospinal fluid and radiological examination are the mainstay for the diagnosis of these diseases. With respect to the acute inflammatory diseases of the brain computed tomography and magnetic resonance imaging fulfill three purposes: 1. They can be used to clarify the diagnosis and to rule out other diseases. 2. They can identify the focus from which a bacterial meningitis can evolve. 3. Complications like edema, cerebral vasculitis, septic sinus thrombosis, hydrocephalus or abscess can be visualized. If the diagnosis is made early, the possible complications are recognized in good time and the appropriate therapy is started immediately, then morbidity and mortality can be kept at a minimum.