It has long been known that the living brains of humans and other mammals generate macroscopic electromagnetic activity that can be recorded from both within and outside the head. From the earliest days of recording this activity, researchers have sought to understand its relation to brain function and to use it to monitor and assess brain state. Continuous records of brain activity, examined without regard to particular points in time, are often useful for determining brain state. However, more detailed knowledge of brain function depends on precise temporal registration of the activity to specific events, either in the external environment or self-generated. The Event-Related Potential (ERP) is a temporal signature of macroscopic brain electrical activity related to a sensory, motor, or cognitive event. The Event-Related Field (ERF) is the magnetic correlate of this activity.