Encephalopathies and epileptic seizures are usually distinct, but important, components of neurologic illnesses. Each may cause altered mental status and behavior, but by different mechanisms. Encephalopathies involve widespread, usually inhibitory, brain dysfunction. Epileptic seizures usually consist of paroxysmal rhythmic excitatory brain activity that produces abnormal function. Some syndromes include both. Sometimes, an encephalopathy may cause seizures, and many patients are encephalopathic postictally. Nonconvulsive seizures and nonconvulsive status epilepticus may be difficult to distinguish from encephalopathies. The EEG is crucial in the distinction, but epileptiform features may appear in either. The greatest area of overlap and difficulty in diagnosis is probably that following clinical seizures, when a patient may have a postictal encephalopathy or may still be seizing.